I'm using Doom in Emacs 27.2. I can call
imenu-list-smart-toggle and I get the imenu window. If I switch to that window using the keyboard, it functions normally.
But, when the cursor is in my text editing window and I use the mouse to click on an imenu window entry, cursor focus jumps back to my editing window without any navigation occuring. I have to click on a blank area in the imenu window to switch focus to that window, and then click on the menu entry.
imenu works fine in my old (Spacemacs) setup so I'm not sure what is going on.