FT search window doesn't go under main Calibre Window easily

When I click the "FT" symbol on the lefthand side of my search bar in the main Calibre GUI, it opens Full-text search dialog window.
I noticed that when I click on the main calibre window, the full-text search window does not go below it. My workaround is clicking the FT search window in the toolbar to make it minimize, or making it go to another workspace (Ctrl-Alt-Shift Right/Left arrow). I wish full-text search window is treated just like any other window.

i use Linux Mint cinnamon.