Modern UI: sidebar sash becomes too wide and off-center with maximized Panel

Does this issue occur when all extensions are disabled?: Not tested yet

- VS Code Version: 1.135.0 (Universal, arm64), commit `08d4889f9ec4a1685d257b9b95de036c8e1ce1e5`
- OS Version: macOS 26.6.2 (Darwin arm64, build 25G83)
- Hardware: Mac mini, Apple M4

## Preconditions

- VS Code's new Modern UI is enabled.
- Panel alignment is `Center`.
- Run `View: Toggle Maximized Panel` so the bottom Panel fills the editor area and the editor area is hidden.

## Steps to Reproduce

1. Maximize the bottom Panel as described above.
2. Hide both the Primary Side Bar and the Auxiliary Bar (Secondary Side Bar).
3. Show only the Primary Side Bar.
4. Observe the vertical sash between the Primary Side Bar and the Panel/editor area.
5. Hide the Primary Side Bar and show only the Auxiliary Bar.
6. Observe the corresponding vertical sash.
7. Show both sidebars at the same time and drag one of the vertical sashes.

## Actual Behavior

- When only one sidebar is visible while the Panel is maximized and the editor area is hidden, its vertical sash is visibly wider than expected and the three-dot grip is not centered in the sash.
- The same issue can occur for either the Primary Side Bar or the Auxiliary Bar, depending on which one is shown first.
- Dragging a sash does not reliably fix the geometry while only one sidebar is visible.
- Once both sidebars are visible, dragging a sash forces a relayout/redraw and the sash width and grip become correctly centered.
- The issue is reproducible with the sequence above; it is not reliably reproducible when the editor area is present.

## Expected Behavior

The sash width, position, hit area, and grip should be correct immediately after showing either sidebar, regardless of whether the other sidebar is visible. The layout should not require both sidebars to be visible and a sash drag to become correct.

## Additional Information

This appears related to, but distinct from, [#324215](https://github.com/microsoft/vscode/issues/324215) and [#326129](https://github.com/microsoft/vscode/issues/326129). It specifically requires a maximized Panel with no editor area and is triggered by toggling sidebar visibility.

A screen recording will be attached in a follow-up comment.