Recently started using the secondary display feature on Lightroom CC, setting it to Photo Grid, while the primary is in detail mode. I'll use the Photo Grid to sort thru similar pictures and reject using the X key.
The issue is that if you maximize the Secondary display, then select images and hit x, the secondary window resizes to smaller than full screen.
Version 8.5.1
Windows 11
Steps to reproduce: In main window, in the bottom icon for Secondary Window, select photogrid. Move secondary window to second monitor and either double click the header or click the maximize icon. Select either 1 or many images in the photo grid. Hit the X key to reject. Secondary window will come out of full screen.