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This change introduces the `accessibility_tree` module, containing code to build an in-memory representation of a very basic accessibility tree for web contents. Currently, the tree for a given document contains: - a `RootWebArea` which has the document root node as its sole child, - an `Unknown` node for the root DOM node, - a `GenericContainer` node for each DOM element, and - a `TextRun` node for each text node. This allows us to make basic assertions about the tree contents in the `accessibility` test by doing a tree walk to find text nodes and checking their contents. Right now, the tree is rebuilt from scratch when accessibility is enabled and when a navigation occurs (via `Constellation::set_frame_tree_for_webview()` sending `ScriptThreadMessage::SetAccessibilityActive`); it's not responsive to changes in the page. This change also changes the way we handle updating the graft node between the webview's accessibility tree and its top level pipeline's accessibility tree. Previously, `Constellation::set_frame_tree_for_webview()` would send a `ConstellationToEmbedderMsg::DocumentAccessibilityTreeIdChange` method informing the webview of the accesskit TreeId of the top-level pipeline. However, this resulted in flaky timing as we couldn't depend on that message being handled before the message containing the TreeUpdate from the WebContents, which would lead to a panic as the new TreeId wasn't grafted into the combined tree yet. This change introduces an epoch value which flows from the ConstellationWebview, where it's updated every time the `active_top_level_pipeline_id` changes, to the layout accessibility tree, and finally to the webview with each TreeUpdate. Whenever a TreeUpdate arrives at the webview which has a newer epoch than the last known epoch, the webview-to-contents graft node is updated before the TreeUpdate is forwarded. If a TreeUpdate arrives at the webview with an epoch _older_ than the last known epoch, it's dropped, as it must be for a no-longer-active pipeline. Fixes: Part of #4344 --------- Signed-off-by: delan azabani <dazabani@igalia.com> Signed-off-by: Alice Boxhall <alice@igalia.com> Co-authored-by: delan azabani <dazabani@igalia.com> Co-authored-by: Luke Warlow <lwarlow@igalia.com>