Match the window size to screen.
When the Agenda/ My Day tab is open, replying to an email somehow forces the window to expand past the edge of the screen and there doesn't seem to be a way to adjust this in the settings.
I believe this issue occurs in more situations than described in the initial post. It does not matter if I am specifically replying to an email. Any time I click the icon to "Show task pane" the window size will change, going beyond the edges of the screen.
This happens to me very consistently too. It happens whether the window is full screen or not. You cannot easily resize it unless you get out of full screen mode.