Browser keyboard diagnostics
Keyboard Tester
Press every key. Keys turn green once they register. The tester also flags key chatter (one press registering twice), counts how many keys register at once, and checks mouse buttons and scrolling.
Keys pressed that are not on the drawn layout (media keys, ISO key, keys hidden by the layout choice) appear here:
Windows keeps a few keys for itself: the Fn key never reaches a web page, Print Screen only registers on release, and Alt+Tab / Ctrl+Alt+Del cannot be blocked. A key with an orange outline registered twice for one press, which usually means a worn switch or a dirty contact.
Mouse & touchpad
Unwanted double clicks on a single press point to a failing mouse switch.