[texshop] Minor bug in TeXShop's preview toolbar

[koch at uoregon.edu]

Bruno,

I can verify your bug report, at least on Catalina. The tool you are using is available in both double window mode, on the preview window, and single window mode. If you select it in single window mode, then it vanishes on double window mode. 

This is related to a similar bug in the early days of Mojave, and I should have a fix later today.

Dick Koch