[Windows] Desktop app: caption close button overflows the title bar and covers the Background tasks panel's X - clicking it quits the app

What happens

On the Windows desktop app, the window's caption close button is drawn taller than the title bar itself and bleeds into the content area below it. When the Background tasks panel is open, that overflow lands directly on top of the panel's own close (X) control.

The result: moving the cursor toward the panel's X lights up the big red window-close hover state instead, and clicking there quits the entire app, killing every running session and background subagent.

Measurements

Taken from a full-screen capture at the moment of hover (3840x2160 device px, 1280x720 logical — 300% display scaling):

| | device px | logical px |
|---|---|---|
| Title bar height (to bottom border) | 173 | ~58 |
| Caption close button hover box | 272 x 220 | ~91 x 73 |
| Caption button overflow past the title bar | 46 | ~15 |
| Panel's X control, center | y 236 | y ~79 |
| Vertical gap: bottom of red box → panel X center | 17 | ~6 |

Two things are wrong here:

1. The caption close button is oversized. ~91 x 73 logical px against the Windows standard of 46 x 32 — roughly 2x wider and 2.3x taller.
2. It overflows its own bar. The hover box extends 46 device px (~15 logical) below the title bar's bottom border, into content that belongs to the page.

Because the panel's X sits at the same horizontal position, the two controls end up vertically stacked with only ~6 logical px of separation. Roughly 6 pixels of mouse travel is the entire difference between "dismiss this panel" and "quit Claude".

Steps to reproduce

1. Windows, display scaling at 300% (likely reproduces at 200% too, with a smaller margin).
2. Run something that spawns subagents so the Background tasks panel appears at the top right.
3. Move the cursor toward the panel header's X.
4. The window caption close button lights up red on top of it.
5. Click → the whole app exits.

Expected

• The caption close button should not paint or hit-test outside the title bar's own bounds.
• Its hover box should be sized to the platform norm rather than ~2x it.
• Failing both, the Background tasks panel's close control should be inset far enough that it can never fall inside the caption button's hit region.

Impact

High. A single misclick on a control the user is deliberately aiming at terminates the app and every in-flight background agent, with no confirmation.

Environment

• Claude Code 2.1.229, entrypoint claude-desktop
• Windows 11 Pro 10.0.26200
• 3840x2160 display at 300% scaling