Starting a few days ago, mouse-clicks are ignored on many modal dialogs or the dialog does not open. For example, when I open a Printers & Scanners ->Printer dialog, I cannot click to choose the scanner.
Many file-upload dialogs in Chrome or Firefox do not open, or if they do, the mouse-click is ignored, and I must use the keyboard to tab to the file in the file selector.
Clicking to "print" to a PDF from the printing dialog in Firefox causes the GUI to freeze.
What could be causing this? How can I diagnose it?
BigSur 11.1.6 on Macbook Pro 2017