I am using an iMac Pro 5120 × 2880 res, with a secondary 1920x1080 monitor. When premiere is open on my secondary monitor, everything works fine, except trying to attach proxy files. When attempting to attach proxies the window that pops up is tiny and almost unusable. I have to stretch it to fit the whole screen only to get about 1/8 of screen value. When I drag premiere over to my main monitor on the iMac Pro and open the proxy menu it works normally. Trashing preferences resolves issue momentarily but will eventually come back. a Work around I found is to undock all windows and keep the top bar with home button and workspaces open on iMac monitor while all other windows are on my secondary monitor. This tricks premiere into thinking that premiere is still open on my main monitor and will open the proxy window normally.