Anchoring-first responsive layout for TUI applications.

Anchoring-first means:
- Terminal resize must not break layout, focus, or rendering
- Controls maintain stable spatial relationships
- Predictable expansion/shrinking rules

Default: Left + Top
→ Position stays stable, size constant

Left + Right:
→ Width stretches/shrinks with parent width

Top + Bottom:
→ Height stretches/shrinks with parent height

All four (Left + Top + Right + Bottom):
→ Rect stretches in both directions

No Anchors:
→ Treat as Left + Top (no "floating" ambiguity)

<!-- Fixed position top left -->
<Button Anchors="Left,Top" Text="OK" />
 
<!-- Full width, fixed height -->
<TextEdit Anchors="Left,Top,Right" Height="3" />
 
<!-- Fills entire available area -->
<ListView Anchors="Left,Top,Right,Bottom" />
 
<!-- Status bar at bottom -->
<StatusBar Anchors="Left,Right,Bottom" Height="1" />

1. Receive resize event
   └─ Update root available rect

2. Invalidate layout
   └─ Perform measure/arrange pass

3. Preserve focus
   └─ Same logical control keeps focus

4. Re-render
   └─ Paint → DiffFlush

REQUIRED:
- Same input → Same layout
- No flicker during rapid resize
- No cursor corruption
- Focus stays on logical control

Optional but recommended resize thresholds:

<Panel ResponsiveMode="Auto" Breakpoints="80,120">
  <!-- Automatic reflow when breakpoint crossed -->
</Panel>

REQUIRED:
- Breakpoints NOT "magic" - define in PXAML or theme config
- Document breakpoints
- Clearly define behavior at each breakpoint

Orientation: Vertical or Horizontal
Children are arranged sequentially

<Stack Orientation="Vertical">
  <Label Text="Name:" />
  <TextEdit />
  <Label Text="Email:" />
  <TextEdit />
</Stack>

Positions: Left, Top, Right, Bottom, Fill
Children dock to sides, remainder fills center

<Dock>
  <MenuBar Dock="Top" />
  <StatusBar Dock="Bottom" />
  <Sidebar Dock="Left" Width="20" />
  <Content Dock="Fill" />
</Dock>

For popups, modals, tooltips
Overlays other controls
Must stay within screen bounds (clamped)

<Overlay>
  <Dialog X="10" Y="5" Width="40" Height="10">
    <!-- Modal content -->
  </Dialog>
</Overlay>

Define rows and columns
Place controls in cells

Order of layout calculation:

1. Container layout (if explicit)
   └─ Stack/Grid/Dock/Overlay calculates child rects

2. Apply anchors
   └─ Refine within container rect
   └─ Respect min/max
   └─ Preserve margins/padding

3. Clipping last
   └─ Clip final bounds

PREFERRED:
- Use ScrollViewer container
- Make content scrollable

NOT as default:
- Text "shrink to fit"

REQUIRED:
- Controls can render in smaller rect (clip + ellipsis)
- State must not be lost

<ScrollViewer Anchors="Left,Top,Right,Bottom">
  <Stack Orientation="Vertical">
    <!-- Long list of controls -->
  </Stack>
</ScrollViewer>

REQUIRED:
- Focus tracked by control identity (not screen position)
- After reflow:
  - If focused control still focusable and visible → keep
  - Otherwise: Fallback to next focusable sibling in focus order

procedure TLayoutEngine.ApplyReflow;
var
  PreviousFocus: TWvdSControl;
begin
  (* Remember focus *)
  PreviousFocus := FocusManager.FocusedControl;
 
  (* Recalculate layout *)
  MeasureAndArrange(RootControl);
 
  (* Restore focus *)
  if (PreviousFocus <> nil) and
     PreviousFocus.CanFocus and
     PreviousFocus.IsVisible then
    FocusManager.SetFocus(PreviousFocus)
  else
    FocusManager.FocusNextAvailable;
end;

[ ] Narrow <-> Wide repeatedly: no flicker
[ ] Cursor not corrupt after resize
[ ] Focus stays on same control
[ ] Anchored stretch controls don't overlap
[ ] Popups/Overlays stay within screen bounds

[ ] Layout switches correctly when breakpoint crossed
[ ] No flicker during breakpoint switch
[ ] All breakpoints documented

[ ] Stack arranges children correctly
[ ] Dock fills correctly (Left, Top, Right, Bottom, Fill)
[ ] Overlay stays in bounds
[ ] Nested containers work

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