Currently we are not adding split/join corner zones to screens that are SCREENMAXIMIZED or SCREENFULL, since it makes no sense to split those windows.
This patch does a similar thing to screens that are "temp", like when Preferences is opened as a separate window. It makes no sense to open up Preferences in a window and then split it.
But of course this still allows splitting/joining if Preferences is used as a regular editor.
Additionally, if the window is "temp" and only contains a single area then it no longer draws the border (with rounded corners). Having a popup window like Preferences with the rounded corners looks funny. In the same way that we don't round the corners of topBar or statusBar.
The following shows how the popup Preferences looks before and after the patch is applied: