Mail will not maximize

[Jack Desert]

When I open mail, it does not fit on my 1366 x 768 display. Instead, it opens approximately 100 pixels high and 2200 pixels wide. When I click the maximize button in the top right corner, it does nothing.

This is a new problem that began about Jan 1, 2017.

Note I do occasionally use this laptop with an external monitor, which I use with the external monitor rotated 90 degrees. I don't know if that has anything to do with it.