fix: cap the first tab on every pass, before a selection widens it (#748)

selectSession() toggles .active at once while _syncTabStripCorner() only runs in
the debounced render, so a cap set after measuring left a newly selected first
tab wider than the room and dropped the first row below the toolbar until then.
max-width changes nothing on a narrower tab, so the cap is simply always on, and
the clear-before-measure and finish-the-animation loops go. The no-usable-room
test uses 8 tabs: at 40 the scroll fallback fires first and it passed at any
threshold.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture-invariants.md

@@ -1047,7 +1047,7 @@ Tests: `test/terminal-touch-tap.test.ts`, `test/session-swipe-gesture.test.ts` (
 
 **Ledger.** CSS only, on `.tabs-ledger` inside `@media (min-width: 768px)` and the header host: the flat list on an auto-fill column grid with equal cells, mono type and a 3px status bar (`--ledger-bar`: yellow for a waiting alert, red for an action alert or an error, muted for stopped, exited and web tabs) instead of the dot. ⚠️ It must not change the markup (pinned: the ledger's innerHTML equals classic's); the rail, the sidebar and narrower strips show the plain list.
 
-**Classic, wrapped: the L-shaped strip.** Once a classic desktop strip in the header host wraps (`tabs-auto-wrap`, or the manual `tabs-two-rows`), `_syncTabStripCorner()` (app.js) adds `.tabs-corner`: `#headerRight` goes `position: absolute` over the strip's top-right corner and the strip runs the header's full width, so only the row(s) beside the toolbar stop short of it and every later row uses the width under it. A flex line cannot be shortened on its own, so in that state the strip is `display: block` with `inline-flex` tabs flowing around a `::before` float sized from the toolbar's box (`--tab-corner-width` / `--tab-corner-height`); the old `row-gap` (lineage's `--lineage-row-gap` included) becomes the tabs' bottom margin and the bottom padding shrinks by the same amount, so row spacing and lineage routing room are unchanged. ⚠️ `updateTabOverflowMode()` removes `.tabs-corner` BEFORE it measures the wrap (the corner takes the toolbar out of the header's flow, which widens the strip), so the wrap is still judged against the one-row layout and an unwrapped strip looks exactly as before. ⚠️ Flex-only rules do not apply to the strip in that state. ⚠️ A strip that already scrolls (past `--tab-strip-max-height`) keeps the plain wrap, checked on the plain strip before the class goes on: the float scrolls with the tabs while the toolbar stays put, and toggling the class on a scrolled strip lets scroll anchoring move it; a strip pushed past the max-height by the float itself falls back too. A first tab too wide for the room beside the toolbar (the whole first line would drop below it) is trimmed to the room instead (`tab-corner-fit`, `max-width: var(--tab-corner-first-max)`, its name ellipsized): a selected tab is wider (gear and close show), so a fallback here made the whole layout follow the selection. Only a room under `TAB_CORNER_MIN_FIRST_TAB` keeps the plain wrap. ⚠️ The first tab is measured with `tab-corner-fit` cleared, or a room that grew would read the last trim as its width. A `ResizeObserver` on the toolbar re-decides wrap and corner and redraws the connection lines when a reading or the header divider resizes it without a tab render. Scope: classic arrangement, desktop, header host only; state/case/ledger, the sidebar and the vertical rail never get the class (`applySessionListLayout()` drops it when the strip moves). Tests: `test/tab-strip-corner.browser.test.ts` (browser suite).
+**Classic, wrapped: the L-shaped strip.** Once a classic desktop strip in the header host wraps (`tabs-auto-wrap`, or the manual `tabs-two-rows`), `_syncTabStripCorner()` (app.js) adds `.tabs-corner`: `#headerRight` goes `position: absolute` over the strip's top-right corner and the strip runs the header's full width, so only the row(s) beside the toolbar stop short of it and every later row uses the width under it. A flex line cannot be shortened on its own, so in that state the strip is `display: block` with `inline-flex` tabs flowing around a `::before` float sized from the toolbar's box (`--tab-corner-width` / `--tab-corner-height`); the old `row-gap` (lineage's `--lineage-row-gap` included) becomes the tabs' bottom margin and the bottom padding shrinks by the same amount, so row spacing and lineage routing room are unchanged. ⚠️ `updateTabOverflowMode()` removes `.tabs-corner` BEFORE it measures the wrap (the corner takes the toolbar out of the header's flow, which widens the strip), so the wrap is still judged against the one-row layout and an unwrapped strip looks exactly as before. ⚠️ Flex-only rules do not apply to the strip in that state. ⚠️ A strip that already scrolls (past `--tab-strip-max-height`) keeps the plain wrap, checked on the plain strip before the class goes on: the float scrolls with the tabs while the toolbar stays put, and toggling the class on a scrolled strip lets scroll anchoring move it; a strip pushed past the max-height by the float itself falls back too. A first tab too wide for the room beside the toolbar (the whole first line would drop below it) is trimmed to the room instead (`tab-corner-fit`, `max-width: var(--tab-corner-first-max)`, its name ellipsized): a selected tab is wider (gear and close show), so a fallback here made the whole layout follow the selection. Only a room under `TAB_CORNER_MIN_FIRST_TAB` keeps the plain wrap. ⚠️ The cap goes on the first tab on every pass, never only once it measures too wide: `selectSession()` widens a newly selected tab at once and this sync is debounced, so a cap set afterwards leaves the first row below the toolbar until it runs. It changes nothing on a tab narrower than the room. A `ResizeObserver` on the toolbar re-decides wrap and corner and redraws the connection lines when a reading or the header divider resizes it without a tab render. Scope: classic arrangement, desktop, header host only; state/case/ledger, the sidebar and the vertical rail never get the class (`applySessionListLayout()` drops it when the strip moves). Tests: `test/tab-strip-corner.browser.test.ts` (browser suite).
 
 With `classic` nothing is left behind: no headings, no breaks, no inline order, no boxes, no name split, no arrangement class. A stale cached mobile-overview.js degrades every grouping to the flat strip. Tests: `test/tab-triage.test.ts`, `test/tab-clusters.test.ts`.