tab: Keep selected tabs visible in overflowing tab bars

[BeratHundurel]

Fixes #2531.

When the tab bar overflows, opening or selecting a tab can leave it hidden or partially clipped until another interaction.

This change keeps scroll indices aligned with logical tabs, reveals the selected tab when the list or its label changes, and retries scrolling after viewport bounds change. Manual scrolling remains unaffected when selection and layout are unchanged.

Verification

• Added regression coverage for insertion, label replacement, viewport changes, and manual scrolling.
• All 15 tab tests, formatting, and Clippy pass.

Reproduction

The recording below demonstrates the issue in an app I’m developing with GPUI Kit.

Before

before.mp4

After

after.mp4

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)

[huacnlee]

The fixes to logical tab indices and the timing of explicit scroll requests are useful, but we do not want to introduce the behavior shown in the video that keeps the active tab visible automatically.

Users should be able to scroll the tab bar away from the active tab and leave it offscreen. Changes to tab labels, tab count, or layout should not pull the strip back to the active tab. Activating a tab and explicitly requesting that it be revealed is a separate interaction from continuously maintaining its visibility.

Please narrow this PR to #2531:

• Keep the index and layout-timing fixes so an explicit scroll_handle.scroll_to_item(index) reliably reveals the requested tab, including a newly inserted tab.
• Remove the automatic reveal policy based on the selected index, tab count, selected label, or viewport changes. track_scroll() should not implicitly opt applications into that policy.
• If a layout retry is needed, associate it with the caller's explicit scroll request and preserve its target, rather than requesting the active tab on every bounds change.
• Update the tests to verify explicit scroll requests and preservation of manual scrolling, rather than requiring the active tab to stay visible.

I also reproduced two regressions with additional tests: closing an unselected trailing tab, or moving the entire tab bar down by 20 px without changing its viewport size, resets a manually chosen horizontal offset from -100 px to -4 px. Both tests pass on the base revision and fail on this PR. These illustrate why we should keep this change focused on the explicit scrolling API.