toggle fullscreen button for ImageDisplay

[rhaschke]

implementation for #1802

[peci1]

With a single display, it works well. However, it struggles on my multi-monitor setup:

image_maximize.mp4

You can see the behavior is not consistent on multiple retries.

My monitor setup:

xrandr excerpt:

Screen 0: minimum 320 x 200, current 3120 x 2451, maximum 16384 x 16384
eDP connected 1920x1080+1200+1371 (normal left inverted right x axis y axis) 309mm x 173mm
...
DisplayPort-5 connected primary 1920x1200+1200+171 (normal left inverted right x axis y axis) 518mm x 324mm
...
DisplayPort-9 connected 1200x1920+0+0 left (normal left inverted right x axis y axis) 518mm x 324mm
...

The same happens even when I align the top of display 2 with the top of display 3.

[rhaschke]

@peci1, I'm afraid that's an Qt issue (which we cannot resolve).

[peci1]

Okay, it's nothing horrible ;)