Not sure if this is by design, but when "shading" and "unshading" windows (by
double-clicking the title bar), sometimes the bottom of the window rises up,
sometimes the top of the window rolls down, and sometimes both happen so they
meet in the middle.

A particular problem occurs with windows whose bottom edge is below the bottom
of the screen. Double-click the title bar, and sometimes the top of the window
rolls down to where the bottom is: so the entire shaded window is now invisible
and off screen. At this point, the window is also not represented in the task
bar, but it can be recovered by using the switcher to bring the window to focus
(still invisible) and then maximising it using a keyboard shortcut.