[Bug]: The open thread's sidebar row is too faint to spot at a glance #19903

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/web

Steps to reproduce

1. Open T3 Code with several threads in the sidebar (default theme, light or dark).
2. Open one of them.
3. Glance at the sidebar and try to find the thread you are in.

Expected behavior

The open thread's row stands out at a glance, in both light and dark mode, so you can find where you are in the sidebar without reading every title.

Actual behavior

The open row's background is only slightly different from the sidebar behind it. In practice it barely shows: you have to look closely, or read the titles, to find the current thread. Hovering over other rows or multi-selecting rows makes it worse, because those use nearly the same color or exactly the same one.

The row surface comes from rowSurfaceClassName in apps/web/src/components/Sidebar.tsx. The open row gets bg-sidebar-row-active, and the only other change is the title switching to text-foreground. The weight stays font-medium, the same as every other row that isn't faded. The tokens in apps/web/src/index.css ([data-app-sidebar]) are:

• Light: the open row is white on near-white. The step from the sidebar is 1.5 lightness points, which is hard to see on most displays.
• Dark: the open row is an 11% grey tint on black. It's visible, but faint, and only 3 points away from a hovered row.
• Hover and selection: a hovered row falls between the sidebar and the open row, and multi-selected rows use the same white as the open row in light mode. When either is on screen, the open row is even harder to pick out.

Impact

Cosmetic issue

Version or commit

main @ 42da166

Additional context

Possible directions (no strong preference):

• A clearly stronger fill for the open row in both modes, for example a noticeably darker or lighter neutral, or a light tint of the accent color. It should be well apart from both the hover and selected colors.
• A second cue besides the background, such as a left accent bar, an inset ring, or a heavier title weight, so the row still stands out on themes with low-contrast surfaces.
• Custom app themes (--app-theme-sidebar-row-active) would need to follow the same rule.

Related, but not duplicates:

• fix(web): keep the parent thread highlighted while viewing its subagent #15163: the parent thread loses its highlight while you view a subagent (the highlight is missing, not too faint).
• [Bug]: Sidebar does not follow active thread after Cmd+[ / Cmd+] (back/forward) navigation #16534: the sidebar doesn't follow the active thread after back/forward navigation.
• [Bug]: Selected and hovered dropdown rows look identical when adjacent #19208: selected and hovered rows look identical when next to each other, but in dropdowns.