[Bug]: New UI horizontal tab bar unusable on mobile with 4+ sessions — only close button clickable

Description

The legacy (old) UI worked fine on mobile: the vertical sidebar gave each session enough room, and I could switch between sessions reliably.

After the new UI layout was enabled, the horizontal session tab bar at the top does not scale for small mobile viewports. With 4 sessions open, each tab shrinks to a tiny icon-only square, and the active/4th tab area becomes so narrow that only the close (×) button remains clickable. I cannot tap the tab itself to switch to that session.

This is a regression from the old layout and makes the new UI effectively unusable on a phone.

Related but distinct:

• #36936 reports truncated tab titles on desktop.
• #36987 reports the agent/model selector not fitting on mobile.

This issue is specifically about the tab bar itself becoming unclickable once 4+ tabs are open.

Plugins

No response

OpenCode version

v1.18.4

Steps to reproduce

1. Open OpenCode Web on a mobile browser with the new UI layout enabled.
2. Open a project.
3. Create or open 4 sessions so the horizontal tab bar shows 4 tabs.
4. Try to tap the 4th tab to switch to that session.

Screenshot and/or share link

Mobile screenshot showing 4 squeezed icon-only tabs in the top bar, with the close (×) button on the right. The tab area itself is too narrow to tap.
Screenshot

Operating System

Mobile (reproducible on any narrow viewport)

Terminal

Web UI