I'm running Ubuntu MATE 16.04 with the Compiz window manager and have my monitors arranged vertically/one above the other. The lower monitor is a built-in laptop monitor and is set as the primary display. When I try to drag windows from the upper monitor to the lower one, the title bar/window decoration gets 'stuck' on the bottom of the monitor and will not pass down to the lower one. I can bypass this by disabling "Constrain Y" in the Compiz "Move Windows" plugin, but then I am not able to maximize a window on the lower screen - It will maximize on the window above it instead. Plugins such as Grid, to enable corner snapping, ignore the lower monitor entirely and only think in terms of the upper monitor, displaying the preview box on the upper monitor and only snapping to corners on the upper monitor. Everything behaves correctly when the monitors are arranged horizontally.
Status changed to 'Confirmed' because the bug affects multiple users.