Fix tab split movement and fullscreen

[brsbl]

Summary

• Bind Info move actions to the pane that owns the control instead of the globally focused pane.
• Give tab splits real maximize/restore behavior while preserving their split layout and selection state.
• Use Maximize pane / Restore split on web and Full Screen / Exit Full Screen in desktop.
• Keep the desktop Browser's native WebContentsView from covering the arrangement menu and move targets.
• Show the new-tab + only in the focused split pane while reserving its space in inactive panes.
• Keep the Info scrollbar hidden at rest, show it during active scrolling, then hide it after 600 ms of inactivity.

Info and Diff remain fixed throughout; this layer does not make them closeable.

Screenshot evidence

Web captures use Chrome for Testing 151.0.7922.71 at the same 1440×900 CSS viewport (DPR 2) and the same one-file Split tab UX verification fixture. Desktop captures use Electron 41.7.0 / Chrome 146.0.7680.216 with identical 1440×900 native window bounds and the same Browser/Info split. The before revision is exact parent head 35050edb632c1007f14978c6c5e5bfe4dd83a259; the after revision is exact PR head 1dbfc83bd587b98968d6b0a839da9f81751ce3b9.

Info move ownership

Before, Move bottom invoked from Info moves the previously focused Diff pane instead, leaving Info above Diff.

After, Info itself moves below Diff.

True pane maximization

Before, activating fullscreen expands the right panel but leaves both split panes visible.

After, Info fills the right workspace and the preserved Diff pane is hidden until restore.

Runtime-specific copy

Before, the web arrangement menu says Full Screen.

After, web says Maximize pane.

Focused-pane new-tab action

Before, the + stays in the upper Diff pane even though the lower Info pane is focused.

After, the only visible + follows focus into the lower Info pane; the upper row keeps a reserved slot so its tabs do not shift.

Desktop Browser overlay

Before, the native Example Domain view paints over the renderer menu, leaving only the Full Screen row visible.

After, desktop retains Full Screen copy and the complete Move grid renders above the Browser surface.

Info scrollbar final behavior

At rest after a completed scroll, the overflowing Info pane has no visible thumb.

During active scrolling in the same layout, the thumb is visible at the right edge.

Validation

• Capture manifests record the exact revisions and environments above, identical fixtures and viewports, zero rendered error states, corrected pane order, a maximized Info pane, runtime-specific menu copy, focused-pane + ownership, and transparent/active scrollbar states.
• The desktop pair is a native screencapture, not a renderer-only image, so it includes the WebContentsView that caused the clipping.
• Regression coverage in this layer targets pane-bound move actions, maximize/restore persistence, keyboard menu entry, runtime copy, Browser overlay reference counting, focused-pane new-tab ownership, and the 600 ms transient-scrollbar lifecycle.
• Tests, typechecks, lint, and other CI-equivalent checks run on the pull request through GitHub Actions; none were run locally for this screenshot audit.

BB-Thread-ID: thr_q8degf2y66

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