Dialog overlay gutter seam on Windows 11

[mynameistito]

Description

On Windows 11 (maximized or snapped window), there's a pixel gutter on the right and bottom edges where the character grid doesn't divide evenly into the window. Those gutter pixels get painted with the terminal's default background color.

The upstream OSC 11 fix in opentui (anomalyco/opentui#951, shipped in v0.1.100) syncs the terminal background to the renderer's theme color. That fixed the gutter for the main app area. But when a dialog overlay opens, the gutter becomes visible again because the overlay is hardcoded to RGBA.fromInts(0, 0, 0, 150) (black with alpha) instead of using the theme's background color. The gutter pixels show the theme background while the overlay darkens everything else to black, so you get a visible seam at the edges.

The same problem exists on the sidebar overlay in session/index.tsx which uses RGBA.fromInts(0, 0, 0, 70).

Steps to reproduce

1. Open opencode on Windows 11 with Windows Terminal (maximized or snapped)
2. Use a theme with a non-black background (e.g. opencode default)
3. Open any dialog (session list, model picker, etc.)
4. Look at the right/bottom edge of the terminal window

opencode-issue-23021.mp4

Operating System

Windows 11 (OS Build 26200.8039)

Terminal

Windows Terminal v1.24, Terminal Preview v1.25

[github-actions]

This issue might be related to previously closed issues. Please check:

• BUG: terminal background color mismatch on pixel gutter edges #19383: BUG: terminal background color mismatch on pixel gutter edges (closed, filed by same author, also mentions overlay mismatch when opening dialogs)
• Black border/gap on right and bottom edges in Windows Terminal (unused pixel gutter) #18055: Black border/gap on right and bottom edges in Windows Terminal (closed, the original gutter issue)

If the OSC 11 fix resolved the main gutter but the dialog overlay regression is a new/remaining concern not covered by those issues, this report is still valid as a follow-up. Maintainers may want to verify whether this was tracked as a known remaining issue in the above.