fix(react-tunnel): fill the exit before paint when it mounts in a Suspense reveal

[mfal]

What & why

A TunnelExit that mounts in a Suspense reveal stays empty for one painted frame. In mStudio this makes every row of a List whose items load individually jump: a ListItemView renders for one frame without its tunnelled Avatar, Heading and Text (36 px instead of 61 px), then snaps back.

Cause:

• ListItemView renders its exits before the tunnelled children, so the exit's first render finds no entries.
• The entries commit their children in layout effects. The exit is a MobX observer, and it subscribes via useSyncExternalStore in a passive effect.
• For a sync mount, React flushes passive effects before paint, so nothing is visible. For a Suspense reveal or a transition, it runs them after paint.

Fix: until the exit has rendered committed entries, it dispatches a re-render from a layout effect. React processes that update after every layout effect of the commit and before paint. Cost: one extra render per exit mount while it has no committed entries.

Tests

• New: "Content from entry is in the DOM when the exit mounts in a Suspense reveal" reads the exit in the first MutationObserver callback after the reveal commit. It is red on main ('') and green with the fix.
• Changed: "Content is not rendered if removing previously suspended tunnel entry" advanced the fake timers once. That relied on React pre-rendering the suspended siblings (and starting Lazy500's timer) before the advance. The extra sync render moves the pre-render to a later task, so the test now advances in vitest.waitFor until the entries are there. The assertions are unchanged.
• WebKit: the react-tunnel suite passed 5 of 5 runs (27 tests). The List, Label, Heading and Tabs browser tests in components pass (59 tests).
• Firefox not run locally; it fails to launch in my sandbox.
• mStudio members list, with this build dropped into node_modules: 0 empty rows in 2 runs, down from all 20 rows for at least one painted frame.

Checklist

• PR title is a Conventional Commit and matches the base branch above
• pnpm lint is clean (pre-push hook); pnpm affected:test not run in full — see Tests
• Generated code is committed — none affected
• User-facing strings — none
• Docs — no public API change

🤖 Generated with Claude Code

[github-actions]

Coverage Report for ./packages/components/

Status	Category	Percentage	Covered / Total
🔵	Lines	75.25%	1180 / 1568
🔵	Statements	74.81%	1209 / 1616
🔵	Functions	69.94%	270 / 386
🔵	Branches	69.4%	617 / 889

File Coverage

No changed files found.

Generated in workflow #7024 for commit 999728a by the Vitest Coverage Report Action

[github-actions]

🚀 Preview Deployment

Preview environments are ready:

Type	URL
docs	pr-3314.docs.review.flow-components.de
storybook	pr-3314.storybook.review.flow-components.de

Images:

• docs: ghcr.io/mittwald/flow/docs:pr-3314
• storybook: ghcr.io/mittwald/flow/storybook:pr-3314