in servo#42336, we added a
[Servo](https://doc.servo.org/servo/struct.Servo.html)-global API for
controlling whether accessibility is active. the idea was that when the
embedder activates accessibility, all webviews and documents activate
accessibility and start sending AccessKit tree updates, and vice versa
when they deactivate.

we found a problem with this approach in servo#42338. global activation of
accessibility makes it too easy to accidentally cause a panic in
AccessKit, and harder than it needs to be for embedders to learn that
they are responsible for grafting each webview’s subtree into their main
AccessKit tree as soon as accessibility is activated. this is due to an
invariant of the AccessKit subtree API: if a subtree starts sending
updates before the graft node is created, the program panics.

this patch reworks accessibility activation to make it per-webview. by
requiring embedders to explicitly activate accessibility for a webview,
we can communicate the AccessKit invariant via our API docs.

Testing: this patch includes an initial accessibility test in libservo
Fixes: part of servo#4344, extracted from our work in servo#42338

---------

Signed-off-by: Alice Boxhall <alice@igalia.com>
Signed-off-by: delan azabani <dazabani@igalia.com>
Co-authored-by: Alice Boxhall <alice@igalia.com>

in #43029, we defined an accessibility tree for each webview. we create
those trees when accessibility is activated for that webview, then the
embedder can graft it into their main accessibility tree. at the time,
this accessibility tree was empty, but we ultimately want it to contain
the accessibility tree of the documents loaded in the webview at any
given time. to do that, we need to graft the active top-level pipeline’s
accessibility tree into the webview accessibility tree. we should be
able to ignore nested documents inside iframes here, and leave it to
their parent documents to graft those in as part of layout
accessibility.

this patch hooks into the moment in the constellation where we detect
that the top-level document has changed (#43013), and notifies libservo
of the new AccessKit TreeId for the webview-to-pipeline graft node. this
moment covers navigating to a new top-level document and navigating back
or forward, but not the initial document loaded in the webview, so we
also hook into the moment a ConstellationWebView is created, and do the
same for that initial document. now the accessibility tree for a webview
with accessibility activated should look like this:

- embedder’s main tree
  - graft node for webview tree → webview tree
    - ~~graft node for document back in history~~
    - graft node for active top-level document → (nothing yet)
    - ~~graft node for document forward in history~~

Testing: this patch updates the relevant accessibility test in libservo
Fixes: part of #4344, extracted from our work in #42338

---------

Signed-off-by: Alice Boxhall <alice@igalia.com>
Signed-off-by: delan azabani <dazabani@igalia.com>
Co-authored-by: Alice Boxhall <alice@igalia.com>

this patch plumbs the webview accessibility trees (#43029, #43556) into
servoshell. we add a global flag in servoshell, which is set when the
platform activates accessibility and cleared when the platform
deactivates accessibility. the flag in turn [activates
accessibility](https://doc.servo.org/servo/struct.WebView.html#method.set_accessibility_active)
in existing and new webviews.

Testing: none in this patch, but will be covered by end-to-end platform
a11y tests in WPT
Fixes: part of #4344, extracted from our work in #42338

Signed-off-by: delan azabani <dazabani@igalia.com>
Co-authored-by: Luke Warlow <lwarlow@igalia.com>
Co-authored-by: Alice Boxhall <alice@igalia.com>