On transparent or blurred window backgrounds, several agent panel
surfaces and
the multibuffer header rendered drop shadows and fade gradients that
have no
opaque surface to blend into, so they showed up as dark halos or colored
patches:

- agent message boxes and the activity bar (drop shadows)
- diff-hunk Reject/Keep controls (drop shadow)
- the diff review pane (double-painted `editor_background`)
- agent thread-list rows, the conversation title edit affordance, and
sidebar
  project headers (fade gradient)
- the multibuffer buffer header (`editor_subheader_background` + sticky
shadow)

These are skipped when `window_background_appearance` isn't `Opaque`.
Opaque
windows are unchanged; on transparent windows the elements stay
delineated by
their borders and truncate text with an ellipsis instead of fading.

## Screenshots

Before:
<img width="1920" height="1200" alt="Screenshot 2026-06-09 at 3 26 26
PM"
src="https://github.com/user-attachments/assets/7dc9c76c-7de9-4c50-8719-05957d091706"
/>


After:
<img width="1728" height="1117" alt="Screenshot 2026-06-09 at 3 26 29
PM"
src="https://github.com/user-attachments/assets/889a53ad-9f11-41b9-9f51-ffcbd5c37821"
/>


Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed dark shadow and gradient artifacts in the agent panel and
multibuffer headers when using a transparent or blurred window
background