Bug#870269: Firefox: Investigate emitting events for page-tab selection changes prior to events for newly-active document

[Alex ARNAUD]

Package: firefox
Version: 54.0-2
Tags: a11y
Owner: bugs at hypra.fr
User: bugs at hypra.fr
Usertags: hypra
Forwarded: https://bugzilla.mozilla.org/show_bug.cgi?id=1260598

DESCRIPTION FROM UPSTREAM:
"Steps to reproduce:
1. Load multiple pages in a single Firefox window
2. Place caret and/or focus within the documents
3. Using Alt+<page tab index> to switch from page to page

Desired results: An event for the page-tab selection change, followed by 
caret and/or focus events for the newly-active document.

Actual results: Caret and/or focus events for the newly-active document, 
followed by events for the page-tab selection change.

Rationale: Ideally, a screen reader would speak the newly-selected page 
tab and then speak where on that newly-active page the user is. If the 
event ordering were as described in "desired", that would JustWork(tm). 
Because it's not, accomplishing the ideal behavior requires things like 
checking if the page associated with the current caret-moved and/or 
focused event is not the same as the page associated with the previous 
page location. Given how often the caret moves and/or focus changes, 
this is a drag.

I was chatting with Trev in IRC about this:

<@tbsaunde> so, I think basically the cause is that the  UI page tab 
thingy whatever it is doesn't actually get selected until after we've 
changed documents that are being displayed
<@tbsaunde> so naturally the documents events are fired first
<@tbsaunde> so then the question is what can we do about that
<@tbsaunde> I kind of wonder if we can fix this with aria-selected
<@tbsaunde> that is first set aria-selected="true" (is that the actual 
attribute I forget ;p) then switch tabs, then select the page tab 
thingus in the ui

So I'm filing this as an "investigate" request, in case it is doable, 
for when Trev has some spare time.

Thanks!"