This is a follow-up to Bug 361047.

Currently, when a group is expanded, it is not visually clear which images belong to the group. The original idea of changing the background color of such a group does not play well with different themes, and it also does not help with multiple opened groups (then all expanded images will have the same background color).

A different idea would be to reduce the opacity of opened grouped thumbnails to e. g. 50 percent but only(!) while the mouse is (!) over the *main* group thumbnail. This does not affect visibility of the thumbnails during normal usage, it indicates clearly what is grouped, each group can be visualized separately, and it does not interfere with color schemes. Also, it creates the possibility of correctly sorting grouped images in between non grouped ones when the group is opened.

This is slightly different from the idea in comment 76 of the above bug report which requested the opacity reduction when *not* hovering over one of the group. But doing it like that would again create the issue that multiple expanded groups cannot be told apart which would be confusing.

If reduced opacity is not an option or too slow to compute, enlarging the border of all expanded image thumbnails (significantly - like 3..5x as many pixels) only when hovering above the main thumbnail would also be an option.