`_isDesktopWidth()` in boot.js gates every collapse path on
`matchMedia('(min-width:641px)')` — matching where the rail itself becomes
visible. The CSS rules driving the actual visual collapse were nested inside
the workspace-panel block at `@media(min-width:901px)` — a threshold copied
from the right-panel collapse but with no functional reason to apply here.
Behavioural consequence in the 641–900 px band (tablet portrait + small
laptop windows):
- Rail is visible, user clicks the active icon
- JS adds `.layout.sidebar-collapsed` and writes localStorage='1'
- JS sets aria-expanded='false' on the active rail button
- CSS at min-width:901px does NOT apply → sidebar stays at 300 px width
- User sees no visual change; screen reader announces collapsed state for
a sidebar that is still visible; localStorage silently persists
- Resize to ≥901 px later → sidebar suddenly collapses (surprise state)
Fix: hoist the three `.sidebar-collapsed` / flash-prevention rules out of
the workspace-panel @media block and into their own `@media(min-width:641px)`
block. The rail visibility breakpoint, the JS gate, and the CSS gate now
all agree.
`:not(.mobile-open)` is preserved on both selectors so the mobile slide-in
overlay (handled in the `max-width:640px` block) is never targeted — the
new @641 boundary doesn't change that contract.
Verified breakpoint matrix end-to-end (Node harness over real boot.js +
style.css):
Width | JS desktop | CSS applies | Effect
------|------------|-------------|------------
640 | no | no | no-op (mobile overlay)
641 | yes | yes | collapses ✓
700 | yes | yes | collapses ✓
768 | yes | yes | collapses ✓
900 | yes | yes | collapses ✓
1024 | yes | yes | collapses ✓
Regression test added: `test_css_breakpoint_matches_js_isdesktopwidth`
parses boot.js for the `_isDesktopWidth` matchMedia query, walks CSS to
find the @media block enclosing `.layout.sidebar-collapsed`, and asserts
the thresholds match. Locks the invariant so a future refactor can't
re-introduce the asymmetric-band silent-state-leak.
Test counts:
- tests/test_sidebar_collapse_toggle.py: 35/35 pass (was 34, +1 regression)
- Full suite (Python 3.14, local): 5040 passed, 0 failed
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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