Created attachment 121029 [details]
The left side of the image shows a maximized console, the right side shows a left-half tiled Gnome Boxes, to show the hight of the right screen. Gwenview is clearly rendered out of screen.

SUMMARY

In a multi-screen setup, if the screens do not have the same geometry and Gwenview is started on the smaller screen, the window starts too tall so that it is rendered partially off screen (see screenshot).

STEPS TO REPRODUCE
1. Setup two screens, one vertical and one horizontal, such that the vertical one is taller than the horizontal one (I use: 1200x1920 + 1920x1080).
2. On the shorter screen, launch Gwenview - for example, by right clicking the desktop and choosing "Open Wallpaper Image"

OBSERVED RESULT

Gwenview appears on the shorter screen but the window is taller than the screen height and the title bar, menu and toolbar are not accessible.

EXPECTED RESULT

Gwenview window should be rendered fully within the confines of the screen it is shown in.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.16.80
KDE Frameworks Version: 5.60.0
Qt Version: 5.12.2

ADDITIONAL INFORMATION

See attached screenshot, which demonstrates my setup as explained in the repro. The secreenshot is rectangular (produced by Spectacle) so shows a black area that is not displayed by any screen - Gwenview is clearly shown rendered into that no-fly-zone.