I've been having some problem lately with windows "shrinking". They get a wide empty inner border when not in focus, but only if they are not the only window on the workspace, or otherwise maximized (i.e. in a tabbed parent that takes the whole workspace).
It only happens in i3. I also tested it in gnome and apps don't do that.
Examples (I'm using Evince here, but I've seen it happen with Firefox, Thunderbird and others)
Only window on workspace:
Multiple windows on workspace, unfocused. Note the inner border:
Multiple windows, focused:
I know it's not necessarily i3's fault, but any ideas on what could be the culprit?
What happens if you remove the gaps between windows? Just guessing, but maybe the bug simply takes the gap width and uses it for the inner padding? I don't use gaps at all and I've never experienced this issue. (Docs: https://i3wm.org/docs/userguide.html#gaps)