I just did some further investigations. It happens, if the film stripe at the bottom is decreased to its minimum. (dragging the small double arrow down as far as possible). Moving it up some pixels, the issue disappears. See the attached screenshot :
It shows the state flickering appears if ALT-2 is pressed.
edit : it’s possible to force the issue also by having a film stripe with a large height, then pressing ALT-2 (everything is ok) and then reducing the height of the film stripe. Some pixels above the minimum height flickering starts.
The issue also depends on the width of the side panels. It can be forced by pressing ALT-2, then moving the double arrow right or left.
The issue obviously depends on the relationship of the width/height of the image area, the widths of the side panels and the height of the film stripe at the bottom.
second edit :
further investigation shows that the issue appears at the moment the slider(s) for the image area (horizontal or vertical) appear or disappear.
- Adding a slider reduces the width (or height) in the image area available for the image
- Image gets downscaled in accordance with ALT-2 setting
- Image now is shown smaller, the slider is no longer needed, so it is removed
- Image now gets upscaled in accordance with ALT-2 setting
- Image now is shown larger, sliders are needed again, and so on…
That’s the flickering one can observe.