[Spectacle] [Bug 524305] New: Window Under Cursor captures Plasma desktop as fully transparent with fractional scaling on Wayland

https://bugs.kde.org/show_bug.cgi?id=524305

            Bug ID: 524305
           Summary: Window Under Cursor captures Plasma desktop as fully
                    transparent with fractional scaling on Wayland
    Classification: Applications
           Product: Spectacle
      Version First unspecified
       Reported In:
          Platform: Other
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: General
          Assignee: [email protected]
          Reporter: [email protected]
                CC: [email protected]
  Target Milestone: ---

**Summary**

`Window Under Cursor captures Plasma desktop as fully transparent with
fractional scaling on Wayland`

**Description**

### System information

* Fedora Linux 44 KDE Plasma Desktop Edition
* KDE Plasma: 6.7.4
* KWin: 6.7.4
* Spectacle: 6.7.4
* KDE Frameworks: 6.29.0
* Qt: 6.11.1
* Kernel: 7.1.8-200.fc44.x86_64
* Session: Wayland
* Graphics backend: OpenGL / DRM
* GPU: AMD Radeon RX 6800 XT (Navi 21)
* Kernel driver: amdgpu
* Mesa: 26.1.6
* Display configuration: single 2560×1440 monitor
* Fractional scaling tested: 120% and other values above 100%
* 100% scaling works correctly

### Problem

When fractional display scaling is enabled, Spectacle's **Window Under Cursor**
mode cannot correctly capture the Plasma desktop.

If I place the cursor over an empty area of the Plasma desktop and take a
screenshot, the resulting PNG appears completely black in an image viewer.

Further analysis shows that the PNG is actually **fully transparent**, not
simply black. The entire image contains RGBA values equivalent to:

`0, 0, 0, 0`

The problem occurs only when display scaling is above 100%.

At exactly **100% scaling**, the same capture method works correctly and
captures the Plasma desktop normally.

### Steps to reproduce

1. Start a KDE Plasma Wayland session.
2. Use a single 2560×1440 monitor.
3. Set display scaling to a fractional value, for example **120%**.
4. Open Spectacle.
5. Select **Window Under Cursor**.
6. Move the mouse cursor over an empty area of the Plasma desktop, not over an
application window, panel or widget.
7. Take the screenshot.
8. Open the resulting PNG.

### Actual result

The Plasma desktop screenshot is completely transparent.

Technical analysis of affected screenshots shows:

* RGBA image
* RGB values are all zero
* Alpha values are all zero
* the whole image contains effectively only one transparent pixel value

Normal application windows are captured correctly.

### Expected result

Spectacle should capture the visible Plasma desktop, including the wallpaper
and desktop contents, in the same way it does at 100% scaling.

### Important comparison tests

I performed several tests to isolate the issue.

**100% scaling:**

* Window Under Cursor → Plasma desktop: **WORKS**

**Fractional scaling, e.g. 120%:**

* Window Under Cursor → Plasma desktop: **FAILS — fully transparent PNG**
* Current Screen → **WORKS**
* Normal application window → **WORKS**

The Current Screen capture at fractional scaling produces a normal, fully
opaque 2560×1440 screenshot.

This suggests that the final composited screen output is correct and the
problem is specifically related to capturing the Plasma desktop surface/window.

### Clean-user test

I also created a completely new user account to rule out my existing Plasma
configuration.

With the clean user:

* at 100% scaling, desktop capture works;
* after changing the clean user to fractional scaling, the desktop capture
problem appears.

Therefore the problem does not appear to be caused by my existing Plasma
desktop configuration.

### Other troubleshooting performed

I also tested the following:

* rebooted the entire computer;
* updated Fedora/KDE packages;
* removed a custom Plasma widget from the desktop;
* tested with a clean Plasma user profile;
* verified that normal application screenshots work;
* verified that Current Screen capture works;
* checked KWin, Spectacle, portal and PipeWire logs;
* checked AMDGPU/DRM logs.

The issue remains reproducible with fractional scaling.

I did not find:

* GPU resets;
* AMDGPU hangs;
* DMABUF errors;
* framebuffer errors;
* PipeWire failures;
* Spectacle crashes;
* KWin screenshot errors;
* damaged relevant package files.

### Additional observation

KWin exposes separate screenshot methods such as CaptureWindow, CaptureScreen
and CaptureWorkspace.

Since:

* normal window capture works;
* Current Screen works;
* desktop capture fails only with fractional scaling;

this may be related to how the Plasma desktop surface is represented or
exported through the window/surface screenshot path when logical and physical
screen dimensions differ.

For example, at 120% scaling my 2560×1440 physical display corresponds to a
logical Plasma geometry of approximately 2133×1200.

I suspect the Plasma desktop surface is being captured as a fully transparent
surface instead of the final composited desktop background when fractional
scaling is active.

### Reproducibility

**Always reproducible on my system.**

100% scale → works
Fractional scale >100% → fails

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