TUI: scroll renders stray ANSI-like characters on screen intermittently

[vtemian]

Summary

Scrolling a pane (via mouse wheel or keyboard) intermittently leaves stray characters on screen that look like ANSI escape sequence fragments.

Reproduction

Steps observed by reporter:

1. Open a conversation pane with enough content to scroll.
2. Scroll down/up repeatedly.
3. Occasionally, fragments like ;26M5;975;97;26M5;97;26M4;97;26M5;97;26M render on screen.

Expected behavior

Scrolling repaints cleanly with no artifacts.

Actual behavior

Intermittent escape-sequence-like characters appear in the pane.

Context

• The characters resemble SGR mouse-tracking fragments (CSI < btn;x;y M/m) or truncated SGR/cursor-positioning output.
• Relevant subsystems:
  • src/Screen.zig — dirty-rectangle render, clearRect, force_full_clear
  • src/Compositor.zig — drawDirtyLeaves, drawBufferIntoRect, scroll offset handling
  • src/Viewport.zig — scroll_dirty / isDirty logic
  • src/Terminal.zig — synchronized output enable (CSI ?2026h/l), mouse tracking (?1006)
• Missing details:
  • Terminal emulator name and version.
  • Whether it reproduces with keyboard scroll (PageUp/PageDown/Ctrl-U/D) or only mouse wheel.
  • Whether this is a recent regression.