Editor/chat sash & panel alignment bugs in Modern UI layout

## Feedback

> A couple bugs I discovered while complaining to @Joanna Oikawa:
> - The chat panel seems to end higher than the main editor panel
> - The hover sash between the editor pane and the chat panel seems to extend past the 2 panes
> - The grabby 3 dots only show when both side panels are visible and disappear on hover (you know this only if you use a transparent sash color)
> - The sash hover on the modal editors seem unnecessary
>
> Not a bug but I personally think the sash hover is now unnecessary because the gap and 3 dot grabby are enough indication

**Source**: #workbench-refresh-feedback — SteVen Batten on 2026-07-15
**Reactions**: 👍 ×1, 👀 ×1

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## Context

With the Modern UI floating-panel layout, several sash/alignment issues have appeared between the editor area and the chat panel. The bottom edges of the chat panel and the main editor no longer line up, the hover sash region overshoots the two panes it divides, and the 3-dot "grabby" resize affordance is inconsistently shown (only when both side panels are visible, and it disappears on hover). The reporter also suggests the sash hover highlight is now redundant given the gap + 3-dot affordance already communicate the resize handle.

## Acceptance criteria

- [ ] Chat panel and main editor panel bottom edges align in the floating-panel layout.
- [ ] The hover sash region between the editor and chat panels does not extend past the two panes it divides.
- [ ] The 3-dot resize affordance renders consistently (not only when both side panels are visible) and does not disappear on hover.
- [ ] Evaluate removing the now-redundant sash hover highlight on modal editors, given the gap + 3-dot grabby already indicate the resize handle.