Currently fullscreen windows are lowered when they lose focus. This makes sense on a single monitor, to be able to see the window which has focus, but it's very annoying on multi-monitor configurations. On multi-monitor configurations it makes sense to leave a fullscreen window on one monitor and give focus to another window on a different monitor, to do something while watching a video for instance.
Ideally, fullscreen windows shouldn't be lowered to the bottom when losing focus, just below the window which received it, which would automatically solve this problem, since a focused window cannot possibly obscure that fullscreen window when it's on a different monitor. By a quick look through the relevant xfwm code this might be complicated to implement due to fullscreen windows being placed on a fullscreen "layer", but I thought I'd mention it.
A hacky but easy to implement solution would be to just check where the window which receives focus is, and not call clientAdjustFullscreenLayer if it's on a different monitor. The problem with that is that since this adjustment is happening at focus changes, if the next focus change down the line ends up on a window on the same monitor with the fullscreen window, we'll have no way of knowing that it's still there and needs lowering.
I'm not yet familiar enough with the code to suggest something better, so I decided it's not a good idea to submit a patch for the hacky solution, but I might take a stab at figuring this out if I find the time. For now I just commented out the clientAdjustFullscreenLayer altogether, and suits my needs for the time being.
*** This bug has been marked as a duplicate of bug 9051 ***