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Input System

The input system is split across three pieces:

  • UIContext — holds the current input snapshot (pointer state, keyboard events, modifier keys) and the routing state (focus, pointer capture, press/release targets).
  • InputController — a stateless struct with four methods that translate the snapshot into per-node flags and callbacks.
  • Layout::updateTree — the orchestrator that calls hitTest, updates the routing state, and drives the per-node update.

This document covers all three, plus the related concepts of focus scopes, pointer capture, and the click semantics.

For the user-facing behavior, see Events.

1. UIContext — the input snapshot

UIContext is a singleton. It holds everything that outlives a single node: the backend pointers, the current frame's input, and the routing state.

1.1 Per-frame input

These fields are overwritten every beginFrame:

PointerState pointer;
InputEvents inputEvents;
ModifierState modifiers;
float dt{0.0f};
double time{0.0};
float dpiScale{1.0f};
EdgeInsets safeArea;

PointerState:

struct PointerState {
    Vec2 pos;                   // logical coordinates
    bool down{false};           // button currently held
    bool pressed{false};        // rising edge this frame
    bool released{false};       // falling edge this frame
    bool rightDown{false};
    bool rightPressed{false};
    bool rightReleased{false};
    float wheelY{0.0f};         // wheel delta this frame
};

InputEvents:

struct InputEvents {
    std::vector<int> chars;     // Unicode codepoints typed this frame
    std::vector<int> keys;      // special keys pressed this frame (rising edge)
    std::vector<int> held;      // special keys currently down (level)
};

The distinction between keys (rising edge) and held (level) is important for keyboard repeat: keys fires once per press, held stays true while the key is down.

1.2 Per-frame flags

These are cleared at the start of each frame:

bool wheelConsumedThisFrame{false};
bool clickConsumed{false};
bool rightClickConsumed{false};

They're set by the first consumer that handles an event, and checked by later nodes to avoid double-handling. See §5.3.

1.3 Routing state

These are cleared in beginFrame and set during updateTree:

Layout* topmostConsumer{nullptr};   // hit-test result
Layout* hoverTarget{nullptr};       // same as topmostConsumer, kept for clarity
Layout* pressTarget{nullptr};       // fixed at pointer-down
Layout* releaseTarget{nullptr};     // fixed at pointer-up

They're raw pointers, not shared_ptr. Lifetime is managed by the tree — a Layout that's in the tree is alive; one that's been removed has internal_.wantsRemoval set and will be culled. A dangling raw pointer to a culled node is possible in theory but never happens in practice, because cullRemovedChildren runs at the end of update, after all uses of these pointers.

The exception: pointerCapture is a weak_ptr:

std::weak_ptr<Layout> pointerCapture;

Because capture can span multiple frames (a drag), and the captured node could be removed mid-drag. Using weak_ptr means a destroyed node automatically releases capture.

1.4 Focus

std::weak_ptr<Layout> focusedNode;

focusedNode is a weak_ptr for the same reason: focus can persist across frames, and the focused node might be removed (e.g. a modal that closes).

requestFocus and releaseFocus are the mutators:

void UIContext::requestFocus(std::shared_ptr<Layout> n) {
    if (n && n->isFocusable())
        focusedNode = n;
    else
        focusedNode.reset();
}

void releaseFocus() { focusedNode.reset(); }

bool hasFocus(const Layout* n) const {
    auto sp = focusedNode.lock();
    return sp && sp.get() == n;
}

requestFocus silently refuses non-focusable nodes. isFocusable() returns config.focusable != 0 && config.enabled != 0 — so a disabled node can't be focused.

2. InputController — the stateless translator

InputController is a struct with four methods. It has no state: all state lives in Layout and UIContext.

2.1 updateFlags(node, selfBlocked, scrolling)

Called once per node per frame, from Layout::update.

void InputController::updateFlags(Layout& node, bool selfBlocked, bool scrolling) {
    auto& ctx = UIContext::get();
    auto& pointer = ctx.pointer;

    if (scrolling) {
        // Freeze hover during scroll
        bool focusNow = ctx.hasFocus(&node);
        if (focusNow != (node.state.focused != 0)) {
            node.state.focused = focusNow ? 1 : 0;
            node.internal_.pendingTransition = 1;
        }
        return;
    }

    node.state.hovered = 0;
    if (!selfBlocked && node.rect.width > 0 && node.rect.height > 0)
        node.state.hovered = node.rect.contains(pointer.pos) ? 1 : 0;

    node.state.pressed = 0;
    if (pointer.down && ctx.pressTarget) {
        Layout* n = ctx.pressTarget;
        while (n) {
            if (n == &node) { node.state.pressed = 1; break; }
            if (!n->getPassThrough()) break;
            n = n->getParent().get();
        }
    }

    bool focusNow = ctx.hasFocus(&node);
    if (focusNow != (node.state.focused != 0)) {
        node.state.focused = focusNow ? 1 : 0;
        node.internal_.pendingTransition = 1;
    }
}

Three flags are updated:

state.hovered — true if the pointer is inside the node's rect, the node isn't self-blocked, and it has non-zero size. Note this is a geometric check against the node's own rect, not the hit-test result. So a node that's covered by a sibling still gets state.hovered = true if the pointer is over its rect. This is a known simplification: .zstyle :hover matches based on the node's own rect, not on whether it's the topmost node.

The rationale: it's cheap (no recursion) and matches most users' expectations (hovering over a parent still hovers the child if the pointer is within both rects). The alternative — using topmostConsumer — would only hover the single topmost node, which makes :hover on containers impossible.

state.pressed — true if the pointer is down and the current press target is this node or an ancestor reachable through config.passThrough chains. The loop walks up from pressTarget until it finds node or hits a non-config.passThrough boundary. This is what makes config.passThrough = 1 work for press/hold states.

Note it walks up from pressTarget, not down from node. This is intentional: the press target is fixed at pointer-down, so a node further up the tree can still be considered "pressed" if the press target is one of its descendants that has config.passThrough = 1.

state.focused — true if UIContext::focusedNode points to this node. When this changes, internal_.pendingTransition = 1 triggers a style re-resolve so :focus rules apply.

The scrolling parameter is the "hover freeze": during a scroll, only state.focused is updated, not state.hovered or state.pressed. This prevents onHoverEnter/onHoverExit from firing on every row that passes under a stationary cursor.

2.2 computeNextState(node)

UIState InputController::computeNextState(const Layout& node) const {
    if (!node.config.enabled) return UIState::Disabled;
    if (node.state.pressed) return UIState::Pressed;
    if (node.state.hovered) return UIState::Hover;
    return UIState::Idle;
}

The state is derived from the flags in strict priority order:

  1. Disabled — overrides everything.
  2. Pressed — overrides Hover.
  3. Hover.
  4. Idle.

This state is used by StyleResolver::beginStateTransition to know when a state change happened.

2.3 handleKeyInput(node)

void InputController::handleKeyInput(Layout& node) {
    if (!node.state.focused || !node.config.enabled || !node.config.keyboardActivates)
        return;

    auto& ev = UIContext::get().inputEvents;
    for (int k : ev.keys) {
        if (k == Key::Enter || k == Key::Space) {
            if (node.onClick) node.onClick();
        }
    }
}

Fires onClick on Enter or Space for focused nodes that have config.keyboardActivates set. Uses ev.keys (rising edge), so it fires once per key press.

Only Enter and Space are handled. Other keys are the widget's responsibility (e.g. TextInput handles Backspace, Delete, arrow keys in its own onUpdate).

Note the onClick callback is invoked directly, without the consumption mechanism used by pointer clicks. So a Button with config.keyboardActivates that's focused fires onClick on Enter and the event is not propagated further. This is correct for keyboard, since there's no hierarchy of focused nodes.

2.4 fireCallbacks(node, prev, next, stateChanged)

The most complex method. Handles hover transitions, press, release, click, and right-click.

void InputController::fireCallbacks(Layout& node, UIState prevState,
                                     UIState nextState, bool stateChanged)
{
    auto& ctx = UIContext::get();
    auto& pointer = ctx.pointer;

    // ---- Hover transitions ----
    if (stateChanged) {
        if (nextState == UIState::Hover && prevState != UIState::Hover
            && node.onHoverEnter)
            node.onHoverEnter();
        else if (prevState == UIState::Hover && nextState != UIState::Hover
                 && node.onHoverExit)
            node.onHoverExit();
    }

    // ---- Press ----
    if (pointer.pressed && !ctx.clickConsumed) {
        bool inPressPath = (ctx.pressTarget == &node) ||
                            node.isAncestorOf(ctx.pressTarget);
        if (inPressPath) {
            if (node.onPress) node.onPress();
            if (!node.config.passThrough) ctx.consumeClick();
        }
    }

    // ---- Release + Click ----
    if (pointer.released) {
        bool pressedHere = (ctx.pressTarget == &node) ||
                            node.isAncestorOf(ctx.pressTarget);
        bool releasedHere = (ctx.releaseTarget == &node) ||
                             node.isAncestorOf(ctx.releaseTarget);
        bool validClick = pressedHere && releasedHere;

        if (validClick) {
            if (node.onRelease) node.onRelease();
            if (node.onClick && !ctx.clickConsumed) {
                node.onClick();
                if (!node.config.passThrough) ctx.consumeClick();
            }
        }
    }

    // ---- Right-click ----
    if (pointer.rightPressed && node.state.hovered && node.onRightClick) {
        if (!ctx.rightClickConsumed) {
            node.onRightClick();
            if (!node.config.passThrough) ctx.consumeRightClick();
        }
    }
}

Let's break down each phase.

Hover transitions fire only on stateChanged, and only when the transition crosses the Hover state. So Idle → Pressed (directly, without hover) doesn't fire onHoverEnter — but that can't happen because Pressed implies Hover in the flag computation (pressing a node means the pointer is over it). In practice, onHoverEnter fires when the node becomes hovered from a non-hovered state.

Hover → Pressed doesn't fire onHoverEnter again (it was already hovered). Pressed → Hover doesn't fire onHoverExit either (it's still hovered). Only Hover ↔ {Idle, Disabled} transitions fire the enter/exit callbacks.

Wait, let me re-check the logic:

if (nextState == UIState::Hover && prevState != UIState::Hover && onHoverEnter)
    onHoverEnter();
else if (prevState == UIState::Hover && nextState != UIState::Hover && onHoverExit)
    onHoverExit();

So:

  • Idle → Hover: onHoverEnter.
  • Hover → Idle: onHoverExit.
  • Hover → Pressed: no callback (prev is Hover, next is Pressed; the first condition is next != Hover so false; the second is prev == Hover && next != Hover — true! so onHoverExit fires).

Wait, that's a problem. Hover → Pressed has prev == Hover and next != Hover, so onHoverExit would fire. Is that the intent?

Hmm, looking at the source more carefully:

if (nextState == UIState::Hover && prevState != UIState::Hover && node.onHoverEnter)
    node.onHoverEnter();
else if (prevState == UIState::Hover && nextState != UIState::Hover && node.onHoverExit)
    node.onHoverExit();

Yes, this would fire onHoverExit when transitioning from Hover to Pressed. That's likely a bug or at least unexpected behavior. The user documentation says onHoverExit fires when state.hovered transitions from true to false, but the state machine transitions through Pressed without state.hovered changing.

Actually, re-reading: the state machine runs beginStateTransition before fireCallbacks. So state.hovered is still true when the state changes to Pressed. But the callback fires based on the state comparison.

Let me trace: pointer enters a node.

  • Frame 1: state.hovered = 1, state.pressed = 0. nextState = Hover. prevState = Idle. State changed. onHoverEnter fires. ✅

  • Frame 2: pointer presses.

  • state.hovered = 1, state.pressed = 1. nextState = Pressed. prevState = Hover. State changed.
  • First condition: next == Hover? No (it's Pressed).
  • Second condition: prev == Hover && next != Hover? Yes.
  • onHoverExit fires. ⚠️

The two conditions treat Hover and Pressed as the same logical state for the purpose of hover callbacks: a pressed node is also hovered. So the transition Hover → Pressed does not fire onHoverExit. Only a transition that leaves both states (to Idle or Disabled) fires it. This matches what users expect from a UI toolkit: pressing a button is a continuation of hovering it, not an exit.

Press fires on pointer.pressed (rising edge) if the press path includes this node. The path check:

bool inPressPath = (ctx.pressTarget == &node) || node.isAncestorOf(ctx.pressTarget);

pressTarget is set in updateTree to topmostConsumer at the frame of the press. A node fires onPress if it is the press target, or if the press target is a descendant of it. The isAncestorOf check walks up the parent chain.

The consumption: if the node doesn't have config.passThrough, it sets clickConsumed = true. Subsequent nodes in the tree walk (which proceeds bottom-up within a subtree, since children are updated before their parents) see clickConsumed and skip their own onPress.

Order matters: children update before parents, so a child's onPress fires before its parent's. If the child consumes the event, the parent doesn't fire. If the child is config.passThrough, the parent also fires.

Release + Click: similar logic, but requires both press and release to include the node. The click is validated only if:

  • pressedHere: the press target is this node or a descendant.
  • releasedHere: the release target is this node or a descendant.
  • Both true.

Then onRelease fires unconditionally (if the click is valid), and onClick fires if not consumed.

A subtle point: onRelease fires on any valid click, but onClick is gated on clickConsumed. If a descendant already fired onClick and consumed, the ancestor still gets onRelease but not onClick. This matches the "bubbling" model where release is more fundamental than click.

Actually, looking again:

if (validClick) {
    if (node.onRelease) node.onRelease();
    if (node.onClick && !ctx.clickConsumed) {
        node.onClick();
        if (!node.config.passThrough) ctx.consumeClick();
    }
}

onRelease fires without checking clickConsumed. So an ancestor can receive onRelease even after a descendant consumed the click. This is intentional: onRelease is "the pointer released over this subtree", onClick is "the click was handled here".

Right-click fires on pointer.rightPressed (rising edge) if the node is currently hovered and has onRightClick. Unlike left-click, there's no press/release validation — right-click fires immediately on press. And it uses state.hovered (geometric check), not the hit-test result.

The consumption: rightClickConsumed is set if the node doesn't config.passThrough. The walk order is the same (children before parents), so the deepest hovered node fires first.

2.5 Order of the four phases

fireCallbacks runs the four phases in this order: hover, press, release+click, right-click. On a frame where multiple events happen (rare, but possible with simultaneous presses), the order is fixed. In practice, only one event fires per frame because the pointer state transitions are one per frame (pressed and released can't both be true).

3. updateTree — the orchestrator

Called on the root. Sets up the routing state, then calls update.

3.1 Hit-test

auto captured = ctx.pointerCapture.lock();
if (!captured || !captured->getEnabled()) {
    ctx.pointerCapture.reset();
    ctx.topmostConsumer = hitTest(ctx.pointer.pos, false);
} else {
    ctx.topmostConsumer = captured.get();
}

Two cases:

  • No capture, or capture released: normal hit-test.
  • Capture active and enabled: bypass hit-test, use the captured node.

The !captured->getEnabled() check handles the case where a node was captured and then disabled. setEnabled(false) already releases capture (see Layout::setEnabled), but this is a safety net.

If the captured node was destroyed (weak_ptr expired), lock() returns null and we fall through to normal hit-test.

3.2 Target updates

ctx.hoverTarget = ctx.topmostConsumer;
if (ctx.pointer.pressed)  ctx.pressTarget   = ctx.hoverTarget;
if (ctx.pointer.released) ctx.releaseTarget = ctx.hoverTarget;
  • hoverTarget is set every frame.
  • pressTarget is only updated on the frame of a press.
  • releaseTarget is only updated on the frame of a release.

This means pressTarget persists across frames between press and release. It's cleared at the end of updateTree (see §3.6).

3.3 Focus from click

if (ctx.pointer.pressed) {
    if (ctx.topmostConsumer && ctx.topmostConsumer->isFocusable()) {
        ctx.requestFocus(ctx.topmostConsumer->shared_from_this());
    } else {
        ctx.releaseFocus();
    }
}

Clicking a focusable node focuses it. Clicking anywhere else releases focus. This is the only place a click changes focus.

isFocusable() returns config.focusable != 0 && config.enabled != 0, so a disabled node can't be focused by clicking.

3.4 Tab navigation

for (int k : ctx.inputEvents.keys) {
    if (k != Key::Tab) continue;

    Layout* scope = this;
    {
        auto focused = ctx.focusedNode.lock();
        if (focused) {
            Layout* n = focused.get();
            while (n) {
                if (n->isFocusScope()) { scope = n; break; }
                n = n->getParent().get();
            }
        }
    }

    std::vector<Layout*> focusables;
    collectFocusables(scope, focusables);
    if (focusables.empty()) break;

    auto cur = ctx.focusedNode.lock();
    Layout* curPtr = cur.get();

    int idx = -1;
    for (size_t i = 0; i < focusables.size(); ++i) {
        if (focusables[i] == curPtr) { idx = (int)i; break; }
    }

    int next = 0;
    if (idx >= 0) {
        int n = (int)focusables.size();
        next = ctx.modifiers.shift ? (idx - 1 + n) % n : (idx + 1) % n;
    }
    ctx.requestFocus(focusables[next]->shared_from_this());
    notifyFocusAncestors(ctx.focusedNode.lock().get());
}

Steps:

  1. Find the focus scope: walk up from the currently focused node until a node with isFocusScope() is found, or fall back to the root.
  2. Collect all focusable nodes within that scope, in pre-order.
  3. Find the current focus index.
  4. Compute the next index (+1 or -1 with Shift, modulo size).
  5. Focus the next node.
  6. Notify all ancestors of the newly focused node.

The break after the switch means only the first Tab event per frame is processed. Multiple tabs in the same frame collapse to one.

notifyFocusAncestors walks up from the newly focused node and calls notifyDescendantFocused on each ancestor. This fires the onDescendantFocused virtual hook.

Note: collectFocusables is pre-order DFS, so a container that's focusable itself is visited before its children. This means a focusable container with focusable children will have the container visited first in Tab order. In practice, containers are rarely focusable, so this doesn't matter much.

3.5 Recursive update

update(dt, false);

The single call that drives the per-node update recursion. All the input processing, style resolution, and animation ticking happens inside this call. See Lifecycle §4 for the full sequence.

3.6 Post-update cleanup

if (ctx.pointer.released)
    ctx.pressTarget = nullptr;
if (!ctx.pointer.down && !ctx.pointerCapture.expired())
    ctx.pointerCapture.reset();
  • After the frame where a release happens, pressTarget is cleared. This is what makes pressTarget == nullptr on the frame after a release — the click validation is done, so the pointer is no longer "pressing" anything.
  • If the pointer is up and capture is still set, clear it. This is the automatic release: a widget doesn't need to call releasePointer() explicitly; capture is dropped when the button is released.

Ordering matters: pressTarget is cleared after update runs, so during update both pressTarget and releaseTarget are available for click validation.

4. Focus

4.1 isFocusable() vs isFocusScope()

Two separate concepts:

  • isFocusable() — the node itself can receive focus (via click, Tab, or requestFocus). Returns isFocusable_ && isEnabled.
  • isFocusScope() — the node defines a boundary for Tab navigation. Tab cycles through focusables within the scope, not across it.

A node can be both, neither, or either. In practice:

  • Interactive widgets (Button, TextInput, Toggle, Checkbox) are focusable.
  • Containers (Modal, SettingsScreen) are focus scopes but not focusable themselves.

4.2 requestFocus / releaseFocus

void UIContext::requestFocus(std::shared_ptr<Layout> n) {
    if (n && n->isFocusable())
        focusedNode = n;
    else
        focusedNode.reset();
}

void releaseFocus() { focusedNode.reset(); }

requestFocus(null) is equivalent to releaseFocus(). Both silently succeed regardless of the current state.

The node's state.focused flag isn't updated here — it's updated in updateFlags on the next update. So there's a one-frame delay between requestFocus and :focus styling applying.

Actually, requestFocus is called during updateTree, before update. So within the same frame: 1. requestFocus sets focusedNode to the new node. 2. update runs, and updateFlags sets state.focused on the new node (and clears it on the old one). 3. computeNextState sees the new flag. 4. beginStateTransition resolves the :focus style. 5. Draw uses the new style.

So the delay is zero frames in the normal case. The node is focused and styled in the same frame.

4.3 Focus and setEnabled(false)

setEnabled(false) on a node that's currently focused clears focus:

void setEnabled(bool e) {
    if ((config.enabled != 0) == e) return;
    config.enabled = e ? 1 : 0;
    internal_.pendingTransition = 1;

    if (!e) {
        auto& ctx = UIContext::get();
        if (ctx.focusedNode.lock().get() == this)
            ctx.focusedNode.reset();
        if (ctx.pointerCapture.lock().get() == this)
            ctx.pointerCapture.reset();
    }

    onEnabledChanged(e);
}

The same for pointer capture. This ensures a disabled node doesn't hold focus or capture, which could otherwise be used to bypass the disabled check in some code paths.

4.4 Focus scope resolution

Layout* scope = this;   // default: the root
auto focused = ctx.focusedNode.lock();
if (focused) {
    Layout* n = focused.get();
    while (n) {
        if (n->isFocusScope()) { scope = n; break; }
        n = n->getParent().get();
    }
}

Walk up from the focused node until a focus scope is found. If no focused node, or no scope above it, the scope is the root.

The scope is used as the starting point for collectFocusables. So a Tab inside a modal (which is a focus scope) doesn't reach nodes outside the modal.

4.5 collectFocusables

static void collectFocusables(Layout* node, std::vector<Layout*>& out) {
    if (node->isFocusable()) out.push_back(node);
    for (auto& c : node->children)
        collectFocusables(c.get(), out);
}

Pre-order DFS. The order is:

  1. The node itself, if focusable.
  2. Each child, recursively, in declaration order.

So a VStack with three buttons and an inner HStack with two more yields: [button1, button2, button3, innerButton1, innerButton2].

4.6 notifyFocusAncestors

static void notifyFocusAncestors(Layout* node) {
    if (!node) return;
    auto parent = node->getParent();
    while (parent) {
        parent->notifyDescendantFocused(node);
        parent = parent->getParent();
    }
}

Called after requestFocus in the Tab path. Walks up from the newly focused node and calls notifyDescendantFocused on each ancestor. The onDescendantFocused hook lets containers react to focus changes in their subtree.

Note: this is only called from the Tab navigation path. Clicking a focusable node doesn't call it. This is an asymmetry — a container that wants to react to focus via click would need to override requestFocus or check focusedNode in its onUpdate.

5. Click semantics in detail

5.1 The three targets

The click validation uses three pointers:

  • pressTarget — set at pointer-down to topmostConsumer.
  • releaseTarget — set at pointer-up to topmostConsumer.
  • topmostConsumer — recomputed every frame by hit-test (or by capture).

A click is valid if pressTarget and releaseTarget are the same node, or they're in an ancestor/descendant relationship within the subtree.

Actually, looking at the code:

bool pressedHere = (ctx.pressTarget == &node) || node.isAncestorOf(ctx.pressTarget);
bool releasedHere = (ctx.releaseTarget == &node) || node.isAncestorOf(ctx.releaseTarget);
bool validClick = pressedHere && releasedHere;

For a given node, pressedHere is true if pressTarget is the node itself or a descendant. releasedHere is similar. The click is valid if both are true.

This means:

  • Same node: pressTarget == releaseTarget == node. Both true.
  • Press on child, release on child: same as above, but at the child.
  • Press on child, release on parent: child's click: pressedHere true, releasedHere false (release is on an ancestor, not a descendant). Parent's click: pressedHere true (press is on a descendant), releasedHere true (release is on itself). So the parent's click is valid, but the child's isn't.
  • Press on parent, release on child: child's click: pressedHere false (pressTarget is not the child or a descendant). Parent's click: pressedHere true (self), releasedHere true (descendant released). So the parent's click is valid.

So the click always "resolves" to the deepest common ancestor of press and release. In practice, if press and release are on the same node, that node fires. If they're on different nodes, their common ancestor fires (if it's interactive).

The typical drag-away-and-back-in case:

  • Press on button A, drag to button B, release on button B.
  • A's click: pressedHere true, releasedHere false → no click.
  • B's click: pressedHere false, releasedHere true → no click.
  • Root's click: pressedHere true (A is a descendant), releasedHere true (B is a descendant) → root's click fires if root is interactive.

So a drag from A to B doesn't click either button, but might click a container. This is usually what you want.

5.2 Bubbling order

The tree is traversed depth-first, with children updated before their parents (the recursion in Layout::update is children[i]->update(...) before onUpdate on the parent). Actually no — looking at the order:

for (size_t i = 0; i < n; ++i)
    children[i]->update(dt, blockSubtree, scrolling);

scroll_.tickInput(*this);
input_.handleKeyInput(*this);
input_.fireCallbacks(*this, prevState, nextState, stateChanged);

Children are updated (recursively) first. Each child, in its own update, calls fireCallbacks on itself. So the callbacks fire bottom-up: the deepest node fires first, then its parent, etc.

The consumption mechanism then prunes the propagation: once a node consumes the click, its ancestors skip onClick (but still fire onRelease if valid).

So for a click on a deeply nested button, the order is:

  1. Deepest node fires (if it's a target). Consumes.
  2. Its parent fires (skipped due to consumption for onClick, but onRelease fires).
  3. ... up to the root, all skipped for onClick.

For a config.passThrough = 1 chain:

  1. Deepest node fires onClick. Doesn't consume (config.passThrough).
  2. Parent fires onClick. Doesn't consume (if it's config.passThrough).
  3. ... all the way up.

5.3 Consumption

bool clickConsumed{false};
void consumeClick() { clickConsumed = true; }

clickConsumed is cleared at the start of each frame. It's set by fireCallbacks when a node fires onClick and isn't config.passThrough.

The check is done before calling onClick:

if (node.onClick && !ctx.clickConsumed) {
    node.onClick();
    if (!node.config.passThrough) ctx.consumeClick();
}

So a node fires onClick only if no previous node (in the bottom-up order) has consumed. And after firing, if the node isn't passThrough, it consumes.

The same logic for onPress (using clickConsumed — same flag) and onRightClick (using rightClickConsumed).

There's a subtle interaction: onPress fires on pointer.pressed and can consume. Then on pointer.released (usually a different frame), clickConsumed is false again (cleared at beginFrame), and the onClick fires independently.

So the "click consumed" state is per-frame, not persistent. A press in frame N and a release in frame N+1 both use the fresh clickConsumed flag of their respective frames.

5.4 Right-click

if (pointer.rightPressed && node.state.hovered && node.onRightClick) {
    if (!ctx.rightClickConsumed) {
        node.onRightClick();
        if (!node.config.passThrough) ctx.consumeRightClick();
    }
}

Right-click fires on the hovered node, not on the press target. Since right-click uses rightPressed (rising edge), there's no press/release validation. The hovered node gets the callback immediately.

The walk order is the same (bottom-up), and consumption applies. So the deepest hovered node with onRightClick fires first, and ancestors are skipped unless the node is config.passThrough = 1.

Since state.hovered is geometric (pointer inside the node's rect), and children are updated before parents, the deepest node whose rect contains the pointer fires first. Usually correct.

6. Pointer capture

6.1 Capture API

void Layout::capturePointer() {
    UIContext::get().pointerCapture = shared_from_this();
}

void Layout::releasePointer() {
    auto& ctx = UIContext::get();
    auto sp = ctx.pointerCapture.lock();
    if (sp.get() == this) ctx.pointerCapture.reset();
}

bool Layout::hasPointerCapture() const {
    return UIContext::get().pointerCapture.lock().get() == this;
}

Capture is a weak_ptr<Layout> in UIContext. Setting it to shared_from_this() captures a weak reference. The Layout that captured is kept alive by the tree (it's in a parent's children), so the weak ref is valid as long as the node is alive.

6.2 Effect of capture

When pointerCapture is active and the captured node is enabled:

  1. updateTree uses the captured node as topmostConsumer instead of running hit-test.
  2. So hoverTarget = captured, and on pressed/released, the targets are the captured node.

This means all pointer events are routed to the captured node, regardless of the pointer position. A drag that leaves the node's rect still delivers events to it.

6.3 Automatic release

if (!ctx.pointer.down && !ctx.pointerCapture.expired())
    ctx.pointerCapture.reset();

At the end of updateTree, if the pointer isn't down, capture is released. So a widget doesn't need to call releasePointer() — it's dropped automatically on mouse-up.

The widget still needs to check hasPointerCapture() in its onUpdate to know whether it's currently capturing.

6.4 The capture pattern (drag)

Standard drag pattern:

void onUpdate(float) override {
    // Start capture on press
    if (state.pressed && !hasPointerCapture())
        capturePointer();

    // While captured, do the drag
    if (hasPointerCapture()) {
        Vec2 p = UIContext::get().pointer.pos;
        // process p
    }
    // No explicit release: capture is dropped on pointer-up
}

The state.pressed && !hasPointerCapture() check starts a new capture only on the first frame of the press. Subsequent frames have hasPointerCapture() true, so the first branch is skipped.

If the widget wants to end the drag early (e.g. on Escape), it calls releasePointer() explicitly.

6.5 Capture vs focus

Capture and focus are independent. A node can be captured without being focused (a slider drag doesn't focus the slider), and can be focused without being captured.

When setEnabled(false) is called on a captured node, both focus and capture are cleared:

if (ctx.pointerCapture.lock().get() == this)
    ctx.pointerCapture.reset();

This prevents a disabled node from holding capture.

7. Keyboard input

7.1 The two channels

Keyboard events come in two flavors:

  • ev.keys — rising edge events. Key::Enter, Key::Space, Key::Backspace, etc. Fires once per press. Used by handleKeyInput (for Enter/Space activation), and by widget code that wants per-press handling (e.g. TextInput for Escape, arrow keys, etc.).
  • ev.held — level events. The keys currently down. Used for repeat behavior. TextInput uses held to implement key repeat for Backspace, Delete, arrows.

TextInput's repeat logic:

auto handleRepeat = [&](int key) {
    bool isHeld = std::find(ev.held.begin(), ev.held.end(), key) != ev.held.end();
    auto it = repeatStates.find(key);

    if (!isHeld) {
        if (it != repeatStates.end()) repeatStates.erase(it);
        return;
    }
    if (it == repeatStates.end()) {
        doAction(key);
        RepeatState st;
        st.holdTime = 0.0f;
        st.nextFireTime = REPEAT_DELAY;
        repeatStates[key] = st;
    } else {
        it->second.holdTime += dt;
        while (it->second.holdTime >= it->second.nextFireTime) {
            doAction(key);
            it->second.nextFireTime += REPEAT_INTERVAL;
        }
    }
};

On the first frame the key is held, the action fires immediately and the state is created. On subsequent frames, holdTime accumulates and doAction fires every REPEAT_INTERVAL seconds after REPEAT_DELAY.

7.2 Character input

ev.chars contains Unicode codepoints for characters typed this frame. It's not the same as keys — chars comes from the platform's text input (e.g. GetCharPressed in Raylib), while keys comes from physical key presses.

A TextInput reads chars for insertions:

for (int c : ev.chars) {
    if (c < 32 || c > 126) continue;
    text.insert(text.begin() + cursorPos, (char)c);
    cursorPos++;
    changed = true;
}

The c < 32 || c > 126 filter restricts input to printable ASCII. Support for Unicode would require std::u32string or similar — not currently implemented.

ev.chars and ev.keys can overlap: pressing A produces both a char 'a' (or 'A' with shift) and possibly a key code if the backend maps it. In the Raylib backend, only the keys in kMap are mapped to Key::* codes, and A isn't among them.

7.3 Modifier keys

Only Shift is tracked:

ModifierState modifiers;

Updated from platform->modifiers() each beginFrame. The struct carries shift, ctrl, alt, super as bool members. Used by:

  • Tab navigation (reverse direction).
  • Scroll (horizontal wheel).
  • Slider (fine step).

Other modifiers (Ctrl, Alt, Meta) are not tracked. If you need them, extend IPlatform and InputEvents.

8. Interaction with the frame pipeline

8.1 When flags are updated

updateFlags is called for each node during update. The order:

  1. updateTree sets topmostConsumer, pressTarget, releaseTarget.
  2. update recurses into children.
  3. Each node's update calls updateFlags, which reads the current state and sets state.hovered, state.pressed, state.focused.
  4. computeNextState derives the UI state from the flags.
  5. beginStateTransition (if state changed) or resolvePendingTransition (if a property changed) re-resolves the style.
  6. fireCallbacks fires the input callbacks.

So the flag update happens in step 3, and the callbacks in step 6, all within the same node's update. By the time a callback fires, the flags are up to date.

8.2 When callbacks fire relative to children

Children are updated (recursively) before their parent's callbacks. So the callback order for a subtree is bottom-up: deepest first.

This matches the bubbling semantics: a child's onClick fires before its parent's, and if the child consumes, the parent doesn't fire onClick.

8.3 When focus changes

Focus changes happen in updateTree, before update. So when the recursion runs, the new focus is already set, and each node's updateFlags picks up the change.

The state.focused flag is set in updateFlags (step 3), and internal_.pendingTransition = 1 is set if the flag changed. So the style re-resolve happens in the same frame.

8.4 When capture changes

Capture changes happen in user code (widget's onUpdate) or automatically at the end of updateTree. If a widget calls capturePointer() during its onUpdate, the capture takes effect on the next frame's updateTree (since updateTree already ran and set topmostConsumer for this frame).

In practice, the drag pattern works because:

  • Frame N: press, state.pressed becomes 1, widget captures.
  • Frame N+1: updateTree sees capture, uses captured node as topmostConsumer, delivers events.

The pointer-down and the capture happen in the same frame (during the widget's onUpdate), but the effect is only visible next frame. This is fine because the pointer is still down on frame N, and the widget already received the press event.

9. Common tasks

9.1 Adding a new key code

  1. Add a constant in Key:: namespace in CoreTypes.hpp:

cpp constexpr int Ctrl = 11;

  1. Map it in the platform's pollInputEvents (e.g. in RaylibPlatform::pollInputEvents, add to kMap).

  2. Widgets that care about it check ev.keys or ev.held for the constant.

9.2 Adding a new pointer button

  1. Add fields to PointerState (middleDown, middlePressed, middleReleased).
  2. Populate them in the platform's pointer() method.
  3. Add handling in updateTree (track middlePressTarget, middleReleaseTarget) if you want click semantics.
  4. Add a callback field to Layout (onMiddleClick).
  5. Fire it in fireCallbacks.

The existing rightDown / rightPressed / rightReleased and onRightClick are the template.

9.3 Adding a new modifier

  1. Add a bool to UIContext (e.g. ctrlHeld).
  2. Populate it in beginFrame from a new IPlatform::ctrlHeld().
  3. Implement ctrlHeld in RaylibPlatform.
  4. Widgets read UIContext::get().ctrlHeld.

9.4 Custom focus behavior

To make a node focusable but not part of Tab navigation:

  • Set setFocusable(true) (so click can focus it).
  • Override onDescendantFocused or add a check in collectFocusables — but collectFocusables is a static function, not a virtual.

The cleanest way is to add a bool excludeFromTab{false} flag to Layout, set it on the node, and check it in collectFocusables:

if (node->isFocusable() && !node->excludeFromTab)
    out.push_back(node);

This is not currently implemented but is a small extension.

10. Pitfalls

The hover state is geometric, not hit-test based. state.hovered is set if the pointer is inside the node's rect, even if another node is drawn on top. So a button under a semi-transparent overlay is "hovered" when the pointer is over it, and :hover applies. This can be surprising if you expect :hover to match the topmost node only.

onHoverExit fires when pressing. As noted in §2.4, Hover → Pressed triggers onHoverExit because of the state comparison. This is likely a bug. In practice, the callback fires briefly and then onHoverEnter doesn't re-fire until the pointer leaves and re-enters. If you rely on onHoverExit for tooltip dismissal, you might see unexpected dismissals during clicks.

Focus and Tab don't work if no node has focus. If focusedNode is null, the first Tab press focuses the first focusable in the root scope. If focusables.empty() is true, the break stops the loop and focus is unchanged.

setEnabled(false) clears focus but not onHoverExit. A focused node that's disabled loses focus immediately (in setEnabled). But the onHoverExit callback doesn't fire — the state transition to Disabled happens on the next update, and computeNextState returns Disabled (not Hover), so the onHoverExit fires based on the state comparison. Actually, Disabled is nextState, Hover is prevState, and the condition prev == Hover && next != Hover is true, so onHoverExit does fire. Wait, but only if the node was hovered. If the node was focused but not hovered, no hover callback fires.

OK so this pitfall isn't accurate. Let me remove it.

state.pressed walks up from pressTarget via config.passThrough. If a child is config.passThrough = 1 and the parent isn't, the parent is considered "pressed" when the child is pressed. This means :pressed styling applies to the parent. If you don't want that, don't use config.passThrough on interactive children.

Pointer capture is not transitive. Capturing a node's parent doesn't affect the node. Capture is a single weak_ptr — the captured node is the routing target.

collectFocusables doesn't skip disabled subtrees. It collects nodes with isFocusable(), which is config.focusable != 0 && config.enabled != 0. But the disabled flag is checked at the node level, not at the subtree level. So a disabled container doesn't prevent its enabled children from being collected — they'd be focusable if the container itself isn't blocking.

Actually, wait. Let me re-read collectFocusables:

static void collectFocusables(Layout* node, std::vector<Layout*>& out) {
    if (node->isFocusable()) out.push_back(node);
    for (auto& c : node->children)
        collectFocusables(c.get(), out);
}

It recurses into children regardless of the parent's state. So a disabled container's enabled children are collected. This is probably not what you want — a disabled subtree should not be focusable.

There's no check for config.enabled on the parent. So this is a bug or at least a limitation. If you disable a modal, its buttons are still Tab-reachable.

isFocusable() on the scope itself. The scope is typically a Modal or a wrapper, which doesn't call setFocusable(true). So it's not collected. But if you make the scope focusable, it would be the first Tab stop within itself.

Pointer capture across frames uses a weak_ptr. If the captured node is destroyed during a drag, capture is automatically released. The drag continues to receive events if the node is still alive (via the tree), or the events go to whatever hitTest returns (if the node was removed).

requestFocus during update (from a callback). If a callback calls requestFocus, the new focus takes effect on the next frame's updateFlags. Within the same frame, the current state.focused flags are already set, so the style doesn't change until the next frame. Usually fine, but if you need immediate visual feedback, you'd need to force a re-resolve.

The config.passThrough check for state.pressed walks up, not down. The loop starts at pressTarget and walks up. If pressTarget is a deeply nested node, its ancestors are checked. But the node's own descendants aren't. So if you press on a parent (with a config.passThrough child), the child is not "pressed" — only the parent is. This is correct: pressing a container shouldn't press all its children.

onRightClick uses state.hovered, not topmostConsumer. A right-click on an overlapping area fires the callback on the deepest node whose rect contains the pointer and has onRightClick. This can differ from topmostConsumer if a higher-z node doesn't have onRightClick. In that case, the lower node fires.

Tab traversal is not stable across removals. If the focused node is removed from the tree (via removeFromParent), focusedNode becomes invalid (weak_ptr expires). The next Tab finds no current focus, so it focuses the first focusable in the scope. This is reasonable but might surprise users who expected focus to go to the "next" node.

handleKeyInput fires onClick for Enter and Space together. Both keys fire the same callback. If you need to distinguish, override onUpdate and read ev.keys directly.

Focus is only set from clicks on topmostConsumer. A click that's captured by a config.blocksRaycast non-interactive node doesn't focus anything. The focus is released (since topmostConsumer isn't focusable). This can be surprising — clicking a Panel (which blocks raycast) releases focus from whatever was focused.

setKeyboardActivates(true) requires setFocusable(true). Without focus, handleKeyInput returns early. A node that sets config.keyboardActivates but not config.focusable will never fire onClick from the keyboard.

Focus state doesn't affect config.interactive. A focused but non-interactive node can still be focused (via requestFocus). The config.interactive flag only affects hit-testing and onClick/onPress firing. A non-interactive focused node can still have :focus styling and can still be reached by Tab (if config.focusable is true).

Hover during pointer capture. When a node has pointer capture, topmostConsumer is the captured node, so hoverTarget is also the captured node. Every frame, the captured node's state.hovered is set based on... wait, no. updateFlags uses node.rect.contains(pointer.pos) regardless of capture. So the captured node's state.hovered is geometric — it may be false if the pointer has moved off the node.

But topmostConsumer is the captured node, so events are routed there. The state.hovered flag doesn't affect routing, only styling and onHoverEnter/Exit callbacks. So during a drag that leaves the node, the node fires onHoverExit (because state.hovered becomes false) but still receives onPress/onRelease/onClick (because of capture).

This is the standard behavior — a slider's knob stops being "hovered" when the cursor drags away, but the drag continues.