[Spectacle] [Bug 524379] New: Rectangular Region: overlay and saved capture show the other monitor's content on dual-monitor Wayland (no rotation, no fractional scaling)

https://bugs.kde.org/show_bug.cgi?id=524379

            Bug ID: 524379
           Summary: Rectangular Region: overlay and saved capture show the
                    other monitor's content on dual-monitor Wayland (no
                    rotation, no fractional scaling)
    Classification: Applications
           Product: Spectacle
      Version First 6.7.4
       Reported In:
          Platform: Arch Linux
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: General
          Assignee: [email protected]
          Reporter: [email protected]
                CC: [email protected]
  Target Milestone: ---

SUMMARY
On a dual-monitor Wayland session, Spectacle's Rectangular Region mode shows
the wrong monitor's content in the selection overlay: when selecting a region
on the secondary monitor (DP-2), the overlay displays the image of the other
monitor (HDMI-A-2) instead of the screen's own content.

This is not only a preview problem: the SAVED capture also contains the wrong
monitor's content (the region is cropped out of the wrongly-assigned image),
so Rectangular Region mode is completely unusable on this setup.

Neither output is rotated and both are at scale 1 (no fractional scaling),
so this is distinct from the rotated-output case in bug 520415, though it
looks closely related. Bug 512835 describes the same symptom on X11 with
Intel/AMD graphics.

STEPS TO REPRODUCE
1. Dual-monitor Wayland session, side by side, different resolutions:
   - HDMI-A-2: 3840x2160@120, position 0,0, scale 1 (LG OLED TV)
   - DP-2: 2560x1440@240, position 3840,720, scale 1 (ASUS PG27AQDM)
2. Trigger "Capture Rectangular Region"
3. Try to select a region on DP-2

OBSERVED RESULT
The overlay on DP-2 shows HDMI-A-2's content (its wallpaper), making
selection blind. Completing a selection anyway saves an image containing
HDMI-A-2's content, not what is visible on DP-2.

EXPECTED RESULT
Each monitor's overlay shows that monitor's own content, and the saved
region matches what was selected.

RULED OUT BY TESTING (each tested independently, symptom identical):
- HDR: disabling HDR on both outputs changes nothing (also reproduces with
  HDR enabled on both).
- Vertical offset: moving DP-2 from 3840,720 to 3840,0 (top edges aligned)
  changes nothing. (Reverted afterwards.)

WORKS CORRECTLY ON THE SAME SETUP (suggesting compositor-side capture is
fine and the image-to-screen assignment in the overlay is at fault):
- "spectacle -b -f -n" (non-interactive full workspace): the stitched
  6400x2160 image has every output's content in the correct place.
- "spectacle -b -a -n" (active window): correct.
- Flameshot 14.0.0 (portal full-workspace grab + its own selection overlay):
  region selection works correctly.
- No QML errors or warnings on stderr when launching "spectacle -r".

SOFTWARE/OS VERSIONS
Operating System: CachyOS (Arch-based), kernel 7.1.8
KDE Plasma Version: 6.7.4 (spectacle 6.7.4, kwin 6.7.4, plasma-workspace 6.7.4)
KDE Frameworks Version: 6.28.0
Qt Version: 6.11.1
Graphics Platform: Wayland
GPU: NVIDIA GeForce RTX 5090, proprietary driver 610.57.04

ADDITIONAL INFORMATION
Possibly related: bug 520415 (NVIDIA, rotated output - but no rotation
here), bug 512835 (same symptom on X11 with Intel and AMD GPUs - so likely
not NVIDIA-specific). Happy to provide screenshots, drm logs, or test
patches/git builds on this setup.

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