I have a 2015 Macbook Pro Retina with an external 1920x1200 monitor connected. Since the Thunderbird update to v45, if I drag the mail or message windows from my laptop screen to the external monitor, the window does not display correctly. It shows the whole window but then a "copy" of the same window overlaid at 1/4 size in the upper left corner of the main window. Rolling back to the previous version of Thunderbird seems to fix the problem.