One of the intentions of :focus-visible is to not match if someone mouse-clicks on a div (or other generic element) with a tabindex. The reason is because developers often build custom controls using generic elements and when they unexpectedly show a focus ring on mouse click these same developer reach for :focus { outline: none; }
which we really want to discourage.
The spec attempts to explain that :focus-visible should not match when you mouse click an element with a tabindex, but does so in a way that maybe isn't explicit enough. Quoting from the spec:
If the element which supports keyboard input (such as an input element, or any other element that would triggers a virtual keyboard to be shown on focus if a physical keyboard were not present), indicate focus.
This attempts to highlight the concept of "supporting keyboard input". Noteably a div with a tabindex would not meet this criteria.
If the user interacts with the page via