Self-Review Checklist:
- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior (visual difference only)
- [x] Performance impact has been considered and is acceptable
I was unable to find a specific issue or PR that mentions this.
As Linux installations always use built-in prompts and ignore the `Use
System Prompts` setting (as far as I'm aware) all users using Zed (with
client-side decorations) are affected. The darkened prompt backdrop was
drawn across the full window boundary, including the transparent
decoration/shadow inset, causing it to overlap.
This PR fixes the backdrop to respect the client-side decoration inset
and only applies rounded corners on non-tiled edges. Tiled edges remain
flush, matching the rest of the client-side window decoration behavior.
I've verified it works on Wayland, X11 and causes no regressions on
macOS or Windows (with `Use System Prompts` off).
Notice the screenshots below on a white background showing the before
and after, where the faint overlap is shown broken and fixed (sorry if
you have to squint).
Before:

After:

Release Notes:
- Fixed prompt darkened backdrop/background rendering outside
client-decorated window boundaries on Linux.
---------
Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>