TUI flickers on every agent response/file write; subagent tab content blank until tab switch

Describe the bug

The TUI flickers heavily whenever the agent is thinking/streaming a response or writing files.
During these phases the content area blanks/redraws repeatedly. Additionally, after the agent
works in a subagent tab, the tab content is not shown until I switch back and forth between tabs
— only then does the current work render.

This happens constantly on every chat message, making the output hard to read and interrupting
the flow of reading code + reasoning while the agent works.

Environment

• opencode version: 1.18.22 (upgraded from 1.18.12 — the flicker did not exist in earlier versions
  but has become more frequent with each update since v1.x)
• OS: Windows 11 (x64)
• Terminal: Windows Terminal
• Install method: Chocolatey

Steps to reproduce

1. Start any coding task that makes the agent think / stream a response / edit files
2. Watch the content area while tokens stream or files are written
3. Content flickers/blanks repeatedly during streaming
4. When the agent switches to a subagent, open that subagent tab — its content is blank until
   you switch away and back to the tab

Expected result

Stable rendering during streaming; subagent tab content visible immediately when opening the tab.

Additional context

• Long-running sessions make it worse (matches #37904)
• Similar to #44211 (caret strobes during streaming) but affects the whole content area,
  not just the caret