moveHandler: Remove the monitor grace period timer when a grab ends

[PhilMeyr]

Fixes the GNOME Shell abort reported in #435, which several of us have hit on Ubuntu 26.04 (@YannickMG, @cpaelzer and myself, plus the original report on Debian 13 / GNOME 48).

The bug

_edgeTilingPreview() arms a 150 ms grace period timer when a drag crosses a monitor boundary, so the tile preview sticks to the previous monitor for a moment:

this._latestMonitorLockTimerId = GLib.timeout_add(GLib.PRIORITY_DEFAULT, 150, () => {
    if (timerId === this._latestMonitorLockTimerId) {
        this._monitorNr = global.display.get_current_monitor();
        if (global.display.is_grabbed())
            this._edgeTilingPreview(window, grabOp);
    }
    ...

The callback captures window, but the source is only ever removed in destroy() — never when the grab it belongs to ends. _onMoveFinished()'s finally block tears down everything else (_posChangedId, the tile preview, _isGrabOp) and leaves this one pending.

So if the dragged window is destroyed within those 150 ms — tearing off or dropping a browser tab does exactly that — the orphaned timer still fires and re-enters _edgeTilingPreview(), reaching:

const workArea = new Rect(window.get_work_area_for_monitor(this._monitorNr));

meta_window_get_work_area_for_monitor() → meta_window_get_workspaces() → g_assert_not_reached() on an unmanaging window → SIGABRT. Being a C assertion, no JS guard can contain it, and the whole Wayland session dies.

The unmanaging JS error logged from _onMoveFinished() in every report is a separate, non-fatal event that merely signals the window is gone. In my logs the two are consistently ~140 ms apart, and on 2026-07-28 that error occurred with no abort at all — the session survived two more days.

The fix

• Remove the pending source in the finally block of _onMoveFinished(), so it cannot outlive the operation that armed it. This is what actually fixes the crash.
• Additionally guard the callback with window.get_compositor_private(). global.display.is_grabbed() is a property of the display and says nothing about this window still being managed; the same idiom is already used in focusHint.js:290.

Notes

• Only the grace period path is affected, so monitor-switch-grace-period false works as a user-side workaround on released versions.
• The timer is armed exclusively inside if (this._lastMonitorNr !== currMonitorNr), which matches every report in Wayland crash report #435 being multi-monitor.
• I could not build a deterministic reproducer — it is a race against a 150 ms window — so this is reasoned from four crash traces rather than from a red-to-green test. Happy to adjust if you would rather keep only the finally cleanup and drop the second guard.