Another problem here in Ubuntu 20.04.1 (with Krusader Version 2.7.2), that I don't think I had in 18.04 (with Krusader Version 2.6.0):
I open the advanced search window (Ctrl-S) and find a list of files and folders. Now I can look at one of them in the main Krusader window by double-clicking or using <enter> on it. But if I use Alt-Tab to switch between open windows and 'accidentally' choose the advanced search window and then try to get back to the Krusader main window, the advanced search window 'freezes' itself on top of it.
It's the same problem with WindowsKey-Tab, although this key combination has a slightly different functionality.
I guess that this problem might not be Krusader's fault.