Created attachment 136371 [details]
Screenrecording

SUMMARY

I created a native resolution screenshot of a website which resulted in a 3812x6948 image and viewed it Gwenview.

When I set the zoom to 100% and scrolled down, in the lower quarter of the image a horizontal line appeared. Please see the screen recording.
If I moved Gwenview's window to the secondary monitor, I could not reproduce this issue.

Finally I repeated the steps with a screenshot on the secondary monitor, which resulted in an image with 2532x7636 pixel. Here I could reproduce it on the secondary monitor, though the line appeared at the top just below the navigation of the screenshotted Wikipedia article.
With this picture I could also reproduce it when I moved Gwenview's window to the primary monitor and zoomed out and back in to 100%, though the line appeared at a slightly different position now.

The line could be removed by refreshing the image via F5. The line did not appear when the zoom mode "filled" was used. This issue is independent whether the sidebar was viewed or not. 


SOFTWARE/OS VERSIONS
Operating System: Manjaro Linux
KDE Plasma Version: 5.21.1
KDE Frameworks Version: 5.79.0
Qt Version: 5.15.2
Kernel Version: 5.11.1-1-MANJARO
Graphics Platform: X11
Graphics Processor: Mesa Intel® HD Graphics 620

ADDITIONAL INFORMATION
Scaling: 200%, resolution 3840x2160 (primary), 2160x1440 (secondory)