Dragging maximized window across monitors with fractional scaling desyncs geometry and breaks minimize/maximize

Distribution

Linux Mint 22.3 Zena

Package version

Cinnamon 6.6.9

Graphics hardware in use

Intel WhiskeyLake-U GT2 [UHD Graphics 620] / NVIDIA GP108M [GeForce MX150] (i915 driver, Mesa Iris 25.2.8)

Frequency

Always

Bug description

In a dual-monitor setup under X11 with fractional scaling enabled and differing scale factors between monitors, dragging a maximized window from one monitor to another causes the window geometry to break ("descuadrado"). Subsequent attempts to minimize or maximize the window do not work (the title bar buttons become unresponsive or their click hitboxes are offset).

Steps to reproduce

1. Configure two monitors with different scaling factors under Cinnamon (X11):
   • Primary: eDP-1 at 2048x1152, scale 1.0 (100%)
   • Secondary: DP-2 at 1280x1024, fractional scale ~0.75 (yielding virtual 1705x1364)
   • With Muffin experimental features enabled: ['scale-monitor-framebuffer', 'x11-randr-fractional-scaling']
2. Open an application (especially CSD/browser/electron applications like Chrome or Firefox) and maximize it on one of the monitors.
3. Click and drag the window from its titlebar to the other monitor.
4. Notice that the window geometry breaks upon crossing the boundary into the target monitor.
5. Attempt to click the Minimize or Maximize buttons: they are either unresponsive or have an offset click hitbox.

Expected behavior

Dragging a maximized window across monitors should cleanly unmaximize/restore and transition into the target monitor's coordinate space without breaking window boundaries or rendering the minimize/maximize buttons unresponsive.

Additional information

• Workarounds identified:
  • Dragging an individual tab directly from the browser (which detaches into a newly created floating window) moves cleanly to the other monitor without triggering the issue.
  • Manually restoring / shrinking (unmaximizing) the window to a smaller floating size before dragging it across screens works without issues.
  • Using the shortcut Shift + Super + Left/Right moves the maximized window across monitors cleanly without desyncing.
  • Disabling fractional scaling and using font scaling (100% scale on both monitors) avoids the problem entirely.
• Display setup details:
  • Server: X.Org 21.1.11
  • Monitors:
    • eDP-1: 2048x1152+1705+939, scale 1.0
    • DP-2: 1705x1364+0+0, mode 1280x1024, scale 0.7507

Start by reproducing the issue in Cinnamon's Muffin window manager with X11, two monitors, and the stated fractional scaling factors. Trace maximized-window dragging across the monitor boundary and the coordinate conversion that follows. Done means geometry and minimize/maximize button hitboxes remain correct, with regression coverage for differing scale factors.