When trying to “select all” using Ctrl + A on DuckDuckGo, the whole page is selected instead of the contents of the search input field.

It’s caused by having Pointer Location turned on in GNOME:

It seems /usr/libexec/gsd-locate-pointer, when invoked under fvwm without session
manager, will enable “show pointer location when pressing Ctrl key”
even when the underlying value of the dconf/gsettings key is false.
This interferes duckduckgo’s search box, making key shortcuts with Ctrl
prefix (e.g. Ctrl+C for copy) unusable. The issue is gone after we stop invoking this gsd plugin.

— https://www.reddit.com/r/duckduckgo/comments/a4s35t/cannot_use_ctrl_prefix_key_shortcuts_in/