Enum SystemChange 

pub enum SystemChange {
    TextSelectionClick {
        position: LogicalPosition,
        timestamp: Instant,
    },
    TextSelectionDrag {
        start_position: LogicalPosition,
        current_position: LogicalPosition,
    },
    ApplySelectionOp {
        target: DomNodeId,
        op: SelectionOp,
        seat_id: u64,
    },
    CopyToClipboard,
    CutToClipboard {
        target: DomNodeId,
    },
    PasteFromClipboard,
    SelectAllText,
    UndoTextEdit {
        target: DomNodeId,
    },
    RedoTextEdit {
        target: DomNodeId,
    },
    AddCursorAtClick {
        position: LogicalPosition,
    },
    SelectNextOccurrence {
        target: DomNodeId,
    },
    SeatShortcut {
        seat_id: u64,
        target: DomNodeId,
        shortcut: KeyboardShortcut,
    },
    ApplyPendingTextInput,
    ApplyTextChangeset,
    ActivateNodeDrag {
        dom_id: DomId,
        node_id: NodeId,
    },
    ActivateWindowDrag,
    InitDragVisualState,
    SetDragOverState {
        target: DomNodeId,
        active: bool,
    },
    UpdateDropTarget {
        target: DomNodeId,
    },
    UpdateDragGpuTransform,
    DeactivateDrag,
    SetSeatFocus {
        seat_id: u64,
        new_focus: Option<DomNodeId>,
        old_focus: Option<DomNodeId>,
    },
    SetFocus {
        new_focus: Option<DomNodeId>,
        old_focus: Option<DomNodeId>,
        visible: bool,
    },
    ClearAllSelections,
    FinalizePendingFocusChanges,
    ScrollSelectionIntoView,
    ScrollNodeIntoView {
        target: DomNodeId,
    },
    ScrollCursorIntoViewAfterTextInput,
    StartAutoScrollTimer,
    StopAutoScrollTimer,
}

Framework-determined side effects (system changes).

Unlike CallbackChange (from user callbacks), these are determined by the framework’s event analysis: hit tests, gesture detection, focus rules, text selection, keyboard shortcuts, etc.

Both CallbackChange (user) and SystemChange (framework) are processed through exhaustive match on PlatformWindowV2 — adding a new variant causes a compile error in apply_system_change().

Variants

TextSelectionClick

Advance layout animations by an EXACT step, bypassing the wall clock.

Only the E2E surface emits this. A headless scenario cannot sample real time: the same test would land on a different point of the curve on a fast machine than a slow one, so any assertion mid-flight would be flaky. Stepping by a fixed dt makes the trajectory a pure function of how many times this ran.

A SystemChange rather than a direct call because CallbackInfo hands out &LayoutWindow only — mutation is required to go through this channel, which is also where the mutable window actually exists. Process a mouse click for text selection (single/double/triple click).

Fields

position: LogicalPosition

timestamp: Instant

TextSelectionDrag

Extend text selection via mouse drag.

Fields

start_position: LogicalPosition

current_position: LogicalPosition

ApplySelectionOp

Unified selection operation: cursor movement, selection extension, or deletion.

Replaces the old ArrowKeyNavigation and DeleteTextSelection variants. Every keyboard shortcut maps to a single SelectionOp — see its docs.

Fields

target: DomNodeId

op: SelectionOp

seat_id: u64

The SEAT whose key this was (9b-ii-a-i-d-ii-b): the op acts on that seat’s caret. Seat 0 is the primary.

CopyToClipboard

Copy selected text to system clipboard (Ctrl+C / Cmd+C).

CutToClipboard

Cut selected text to clipboard and delete (Ctrl+X / Cmd+X).

Fields

target: DomNodeId

PasteFromClipboard

Paste text from system clipboard at cursor (Ctrl+V / Cmd+V).

SelectAllText

Select all text in focused node (Ctrl+A / Cmd+A).

UndoTextEdit

Undo last text edit (Ctrl+Z / Cmd+Z).

Fields

target: DomNodeId

RedoTextEdit

Redo last undone edit (Ctrl+Y / Ctrl+Shift+Z / Cmd+Shift+Z).

Fields

target: DomNodeId

AddCursorAtClick

Add a cursor at the clicked position (Ctrl+Click). The position will be hit-tested to find the text cursor location.

Fields

position: LogicalPosition

SelectNextOccurrence

Select the next occurrence of the current selection’s text (Ctrl+D). If the primary selection is a cursor, expand it to the word first.

Fields

target: DomNodeId

SeatShortcut

A NON-primary seat’s editing shortcut (9b-ii-a-i-d-ii-b-i): Copy / Cut / Paste / Select-all / Undo / Redo on THAT seat’s focus and caret. The primary’s shortcuts stay the dedicated variants above.

Fields

seat_id: u64

target: DomNodeId

shortcut: KeyboardShortcut

ApplyPendingTextInput

Apply pending text input from platform (keyboard/IME).

ApplyTextChangeset

Apply text changeset (incremental relayout).

ActivateNodeDrag

Activate node drag on a draggable element.

Fields

dom_id: DomId

node_id: NodeId

ActivateWindowDrag

Activate window drag (CSD titlebar).

InitDragVisualState

Set up drag visual state (:dragging pseudo-state, GPU transform key).

SetDragOverState

Set :drag-over pseudo-state on a target node.

Fields

target: DomNodeId

active: bool

UpdateDropTarget

Update current drop target in drag context.

Fields

target: DomNodeId

UpdateDragGpuTransform

Update GPU transform for active node drag.

DeactivateDrag

End drag: clear pseudo-states, remove GPU keys, end drag session.

SetSeatFocus

Move a NON-primary seat’s focus (9b-ii-a-i-d): only that seat’s entry changes - no :focus restyle, no caret / blink timer, no soft keyboard, which all follow the primary’s focus (the text-edit session is one). A primary-seat change is SetFocus.

Fields

seat_id: u64

new_focus: Option<DomNodeId>

old_focus: Option<DomNodeId>

SetFocus

Change focus to a new target (or clear focus if None). Handles: set_focused_node, apply_focus_restyle, scroll_node_into_view, cursor_blink_timer start/stop.

Fields

new_focus: Option<DomNodeId>

old_focus: Option<DomNodeId>

visible: bool

Should this focus be INDICATED - the W3C :focus-visible question.

It travels WITH the change because the handler restyles and regenerates the display list, and the focus ring is emitted by that regeneration. Setting the flag afterwards (which the shell used to do for the keyboard route) means the frame that was just built did not know focus was indicated, so the ring only appeared on whatever repaint happened NEXT - a click, a resize. On the device that read as “Tab does not ring anything until you click something”, while the E2E harness - which happened to set the flag first - showed a ring and passed (device report, 2026-09-01).

ClearAllSelections

Clear all text selections.

FinalizePendingFocusChanges

Finalize pending focus changes (cursor initialization after layout).

ScrollSelectionIntoView

Scroll cursor/selection into view.

ScrollNodeIntoView

Scroll a specific node into view.

Fields

target: DomNodeId

ScrollCursorIntoViewAfterTextInput

Scroll cursor into view after text input (needs relayout first).

StartAutoScrollTimer

Start auto-scroll timer for drag-to-scroll (60Hz).

StopAutoScrollTimer

Cancel auto-scroll timer.

Trait Implementations

impl Clone for SystemChange

fn clone(&self) -> SystemChange

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for SystemChange

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Eq for SystemChange

impl FromIterator<SystemChange> for SystemChangeVec

fn from_iter<T>(iter: T) -> Self

where T: IntoIterator<Item = SystemChange>,

Creates a value from an iterator.

impl PartialEq for SystemChange

fn eq(&self, other: &SystemChange) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for SystemChange