mouse buttons not working with Qt6 apps [SOLVED]

[ExecutorElassus]

I have two apps that are doing some odd things with their UI, both related to using Qt6. My WM is fvwm3, which appears to be using a mix of Qt and GTK for theming and widgets. Here's what I'm dealing with:

With app-office/texstudio-4.8.4-r2, the primary mouse button doesn't work at all. Clicking does nothing. There is sometimes a context menu (ie, the right-click menu) that appears, but only briefly. Using menus only works with the secondary mouse button. Dropdown menus don't work at all.

With app-office/calibre-7.19, using Qt6, none of the scrollbars -- nor any of the UI buttons on the main calibre window nor in the reader application -- respond to clicks with the primary mouse button. The button shows the button-clicked animation, but there's no action performed. Scrolling with the wheel works, and clicking menu items from the right-click menu in the main calibre window also works. Clicking the "Help" button spawns the calibre manual webpage in a web browser, but no other buttons seem able to spawn their respective windows or menus.

The app that's supposed to control theming for Qt6 -- qt6ct -- also doesn't respond to the mouse correctly.

What happened to my mouse under Qt6, and how can I fix it?

UPDATE: So, it turns out I had two conflicting button mappings: one in .xsession that was this: