Editorial: make implicit autofocus algorithm flag explicit

[annevk]

The autofocus attribute algorithm contains this step:

If the user agent has already reached the last step of this list of steps in response to an element being inserted into a Document whose top-level browsing context's active document is the same as target's top-level browsing context's active document, then return.

This is a little magic. It'd be better to give (all) documents a flag (initially unset), set it when you run this algorithm for the top-level browsing context's active document, and also make this algorithm check for it and terminate if it's set. (It doesn't need to be unset.)

Came up in #3488.

(It'd also be good to write some tests since we don't have many autofocus tests at the moment.)

[domenic]

This was fixed in #4763, yay!