The windows don't get maximized to the whole available screen space (excluding the panel, of course) -- there is some unused space between the window and the panel (say, 10 pixels or so). The only thing present in my setup is a horizontal panel on the bottom of the screen, "small"-sized, with some stuff, including the embedded taskbar applet (the dedicated taskbar panel is disabled). I don't know if it is relevant, though -- at least, running a taskbar at the top doesn't fix the problem.
Created attachment 109 A screenshot illustrating the problem
What are the workspace margins?
Left: 0, Right: 0, Top: 0, Bottom: 44. 44? Just set it to 0 and everything got fixed. Curious to know if it was the default value, I started xfce after having .cache .config .themes moved from my home directory. This reverted everything to defaults, and 44 was there by default. Looks quite strange to me. Thank you very much for pointing out the source of the problem! Now it feels cozy and fine. But why 44 instead of 0 is the default seems unclear to me. Btw, I used debian packages from os-cillation.
Meanwhile I checked the source, and it defaults to 0. I really can't tell how you got 44. Resolving the bug then.