Maximize vertically function does not work properly at windows with client-side decorations - window width gets increased when the maximize action is invoked. Horizonal maximization is not affected.
Steps to reproduce:
- Open window manager settings (xfwm4-settings)
- Set up a key shortcut for "Maximize window vertically"
- Open some CSD application (gedit, nautilus, ...)
- Press the shortcut
Window gets vertically maximized AND window width gets increased a little
Window gets vertically maximized, width remains the same
- xfwm 4.12.2 (revision 913dff9)
- Git - master at commit 634b660757dc90fe785536f902beaf06e03deb56 - http://git.xfce.org/xfce/xfwm4/commit/?id=634b660757dc90fe785536f902beaf06e03deb56
This is normal, actually, gtk+ 3.x CSD windows have an additional area around where they draw their own shadows.
When maximized vertically, older versions of gtk+ 3.x would use that area for the windiow itself as they wouldn't draw their shadow anymore when maximized (you can see that even on your video, when maximized, the shadow is gone and the window is larger instead).
This does not happen with gtk+ 3.16. No bug in xfwm4 here.