Have you ever been annoyed by terminal windows not aligning their edges with edges of other windows or screen edges in xfce? Seems like there is an easy fix, if you know where to look!
Why do the windows work like this? The outer size of a terminal window is determined by its inner window size and decorations. The dimensions of decorations depend on used theme. However the width and height of a terminal window cannot be changed smoothly by default, just in steps. For example if you try to resize xfce4-terminal, it will display it’s size in <width>x<height> format. Displayed units show width in number of symbols and height in lines and have to be an integer, not a fraction. The window then sends it’s size, measured in pixels (or points/whatever), to the window manager as a “size hint”.
To avoid this annoying behaviour, simply install “terminator” and in settings, uncheck “window geometry hints”. Then the windows can fill any arbitrary sized grid of window manager’s choice and you get rid of the gaps.
P.S. Terminator is a great application which you should be already using anyway.