Navigating to an anchor, focus, and caret navigation insertion point ordering.

[emilio]

Somewhat related to #7657.

I'm implementing https://bugzilla.mozilla.org/show_bug.cgi?id=277178 and something that came up during the review was the ordering of steps with regards of "sequential focus navigation start point" and the focus move.

So: "sequential focus navigation start point" in Gecko is just the selection anchor point. This has been long-standing behavior and it seems to me it works quite nicely (the anchor point moves with the mouse when the user clicks on stuff, etc...).

Gecko historically first moves the selection anchor, then clears focus on the window, which is the opposite order to https://html.spec.whatwg.org/#scroll-to-the-fragment-identifier. It seems likely for a focus event listener to modify the selection, so moving selection afterwards seems unfortunate.

So, questions:

• Should really the "Move the sequential focus navigation starting point to target." step be after "Run the focusing steps for target, with the Document's viewport as the fallback target."? Seems potentially undesired regardless of the following question as per above.

• Should really the "sequential focus navigation start point" be a different concept from selection?

For now, I think I'm going to keep the pre-existing order (move selection, then focus change), given this area is not particularly interoperable.

cc @smaug---- @annevk