Comment 18 for bug 310752

This happens if at least one windows dimension is larger than the screen resolution (for example, in the screenshots Roman provided you can see that both Firefox and Pidgin are clamped to the maximum possible height because they requested a window size larger than the screen resolution.
(feel free to confirm that assumption with you own tests please)

In GNOME this automatic clamping and automatic enabling of the Maximized state will happen only if *both* window dimensions exceed the screen resolution. Which is still not very nice, and I consider it a bug, since I don't want my windows forced into Maximize mode - I have to manually un-maximize it in case the window size decreases (imagine an image viewer with zoom-out functionality).

I can see that this automaximize function probably has been implemented as a failsafe, in case a window requests an unreasonable initial size, so it will be forced/clamped to screen resolution this way. However, this is not always what the user wants.

Enabling Maximize mode with only one window dimension exceeding screen resolution (=the current xfwm4 behaviour) could be seen as either an even more restrictive "failsafe" - or a bug. I vote for the latter :o)