Link to issue number:

None. This supersedes an earlier attempt at this (#9672) that was reverted (#10538) due to severe performance problems.

Summary of the issue:

Firefox is adding support for out-of-process iframes; i.e. iframes rendered in a separate process to their embedder document. For an out-of-process iframe, the embedder has no knowledge of children in the embedded document and cannot communicate with the embedded document at all. This means that accChild for accessibles in the embedded document fails. This requires some changes to the gecko_ia2 virtual buffer.

Description of how this pull request fixes the issue:

In the gecko_ia2 vbuf:

1. getNVDAObjectFromIdentifier: Use getNVDAObjectFromEvent instead of accChild on the document. This was changed some time ago to use accChild on the document in the theoretical hope that it might be slightly faster, since it doesn't need to go via the parent process. However, this was never proved to be a performance benefit in real terms. The root accessible is in the parent process and can fetch children in all content documents, so using AccessibleObjectFromEvent works as expected.
2. __contains__: If accChild fails, get the embedder iframe and try accChild with the iframe's id. There can be nested out-of-process iframes, so we keep trying this until there are no more iframes in the hierarchy (or until we hit the buffer's root id). We cache iframes because walking iframes is expensive, and when trying to work out what TreeInterceptor an object belongs to, we'll need to query the iframes for each TreeInterceptor.

Testing performed:

1. Loaded a document with two out-of-process iframes in one tab and a document with no iframes in another tab. In the Python console, saved the former document's TreeInterceptor as ti1 and the latter document's TreeInterceptor as ti2; e.g. ti1 = nav.treeInterceptor.
2. Moved the navigator object to a node in the innermost iframe and pressed NVDA+control+z to take a console snapshot.
3. Confirmed that nav in ti1 is True and nav in ti2 is False.
4. Moved the navigator object to a node in the document with no iframes and pressed NVDA+control+z.
5. Confirmed that nav in ti1 is False and nav in ti2 is True.

Also used a build with this change for a while doing some daily browsing with Gmail, GitHub, etc.

Finally, the performance problems that caused #9672 to be reverted occurred when there were several tabs loaded. I tried with 15 tabs open (focusing each to create a TreeInterceptor for it) and did not observe this performance degradation while browsing.

Known issues with pull request:

There will be some additional COM calls for objects in out-of-process iframes. However, the iframes for each object are cached while the object is alive, since getTreeInterceptor will call __contains__ on each TreeInterceptor until a match is found. There don't tend to be many levels of nested iframes, so filling the cache shouldn't be too expensive.

Other avenues explored:

See #9672.

Change log entry:

Changes:

- Support for out-of-process iframes in Mozilla Firefox.