Checking an item off under the Open filter played a scrambled exit: the row shrank away while the checkbox held its full size and got sliced in half by the shrinking row, and the row's contents jumped sideways the instant the animation began.

Root cause

The hide/show wrapper in checklist_item_row_state.dart was an AnimatedCrossFade from the row to a zero-sized SizedBox.shrink. That has two independent defects, both measured rather than inferred:

1. It clips, it doesn't scale. AnimatedCrossFade collapses via AnimatedSize, which lays the child out at its natural size and animates only the box, clipping the overflow. The fixed 44×44 checkbox therefore kept its full dimensions while the band closed over it.

2. The outgoing row is re-laid-out against zero-size constraints. When crossFadeState flips, the row stops being the sizing child and is laid out against the zero-sized second child. On the very first frame, before any size animation has run, the checkbox jumped from x=64 to x=247 and y=28 to y=46. Under the card's loose width constraints the box also collapsed horizontally (366 → 319 → 183 px), pulling the row in from both sides toward Alignment.topCenter.

Fix

New design-system motion primitive SizeFadeCollapse (components/motion/), the exit counterpart to SizeFadeEntrance. It drives reserved height, paint scale and opacity from one tween sharing one anchor (top-start), so painted size equals reserved size on every frame. Nothing is cropped, the box keeps full width, and the row leaves as a single piece.

// before
child = AnimatedCrossFade(
  duration: checklistCompletionFadeDuration,
  sizeCurve: Curves.easeInOut,
  crossFadeState: _showRow ? CrossFadeState.showFirst : CrossFadeState.showSecond,
  firstChild: child,
  secondChild: const SizedBox.shrink(),
);

// after
child = SizeFadeCollapse(
  duration: checklistCompletionFadeDuration,
  collapsed: !_showRow,
  child: child,
);

Behaviour preserved from the old wrapper: reduced motion snaps instead of animating; a collapsing row is IgnorePointered / ExcludeFocused / ExcludeSemanticsed as soon as it starts leaving, so it can neither be tapped nor announced. Its subtree keeps ticking until the collapse finishes, so an in-flight strike-through wipe or checkbox pop is not frozen half-played. The collapse is reversible — unchecking runs the same tween backwards.

duration is required rather than defaulted: task_details_page.dart computes its scroll-anchor delay from checklistCompletionAnimationDuration + checklistCompletionFadeDuration, and a silent default would let the two drift apart unnoticed.

Verification

Geometry across the collapse after the fix — the checkbox now shrinks with the row and stays inside it at every step, and the row's width never changes:

Confirmed visually too, by rendering the animation frame by frame through the in-app screenshot harness (throwaway capture test, not committed).

• 4 new regression tests in checklist_item_row_test.dart, each verified to fail against the old implementation
• 8 new tests for SizeFadeCollapse (geometry invariant per frame, leading-edge anchor, reverse, reduced motion, pointer/semantics exclusion, duration retiming)
• Existing tests that asserted on AnimatedCrossFade.crossFadeState migrated to SizeFadeCollapse.collapsed
• 2542 tests pass (1 skipped), analyzer clean, formatter clean
• READMEs updated (features/tasks, features/design_system), CHANGELOG + flatpak metainfo updated under 0.9.1069

Deliberately not done

SizeFadeEntrance has the same clip-not-scale geometry, so a row entering the list is revealed by cropping rather than scaling — insert and remove are now asymmetric. That is outside the reported bug and changing it is a product call, so it is flagged rather than folded in. Related: checklist_card.dart already carries a comment about AnimatedCrossFade squeezing an outgoing body at progressively narrower widths — the same defect class, worked around there by keeping a width-stable hidden endpoint.

Bead: lotti3-tp0