cua-driver maps the agent-cursor overlay as a fullscreen, always-on-top,
all-workspaces X11 window (save-unders composited). When a computer-use
session ends uncleanly — an agent interrupted mid-capture, a stale target
window, a driver error — that window can be left stuck above every app on
every workspace, wedging desktop input until the app is restarted. This
is the same failure class as the HUD's transparent always-on-top window on
Mutter/X11 (#83473), and it bit a real user: an interrupted capture froze
the desktop, the app had to be force-restarted, and the overlay window was
still mapped fullscreen afterwards.
The overlay is cosmetic (a tinted cursor sprite); the driver, captures,
and synthetic input all work without it. `_cua_no_overlay()` already
defaulted it off on macOS (idle CPU redraw loop, #28152/#47032) and
headless/WSL2 Linux; this extends the same auto-detect to X11 desktop
sessions, where raw X11 stacking has no compositor-owned surface to tear
down with the driver's connection. Wayland keeps the overlay: the
compositor owns the layer-surface lifecycle, so a dead driver cannot leave
a stuck top window.
Behavior contract unchanged: an explicit `computer_use.no_overlay: false`
still restores the cursor on any platform, and `true` forces it off.
Tests: X11 (DISPLAY set, no Wayland env) and XDG_SESSION_TYPE=x11
auto-detect off; Wayland keeps the overlay; explicit false overrides
auto-detection on X11.
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