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?