editor: Fix opaque multibuffer header backgrounds not rendering in transparent themes

[amtoaer]

Objective

As part of the transparent-window artifact fixes in #58981, multibuffer header backgrounds stopped rendering whenever the window was transparent or blurred.

This caused a regression for transparent themes, which commonly use an opaque multibuffer header background to prevent editor text from showing through sticky headers.

Solution

Render editor_subheader_background when either the window or the configured background color is opaque.

This restores opaque multibuffer header backgrounds as a mask for the editor content beneath them, while preserving the #58981 behavior for translucent backgrounds that would otherwise create a darker layered region.

Testing

Manually verified locally with a transparent theme that an opaque multibuffer header background is rendered and prevents editor text from showing through sticky headers.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Showcase

Nightfox - opaque / blurred

Before:

After:

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Release Notes:

• Fixed editor text showing through multibuffer headers and overlapping header text in transparent themes.

[ChristopherBiscardi]

thanks!