Improve inter-document focus handling (#36649)

*Describe the changes that this pull request makes here. This will be
the commit message.*
rewritten the PR #28571
Implement
[Window#focus](https://html.spec.whatwg.org/multipage/#dom-window-focus),
[Window#blur](https://html.spec.whatwg.org/multipage/#dom-window-blur)
Testing: WPT
Fixes: #8981 #9421

---------

Signed-off-by: kongbai1996 <1782765876@qq.com>
Co-authored-by: yvt <i@yvt.jp>

components/shared/constellation/from_script_message.rs

@@ -15,7 +15,8 @@ use base::id::{
 use canvas_traits::canvas::{CanvasId, CanvasMsg};
 use devtools_traits::{DevtoolScriptControlMsg, ScriptToDevtoolsControlMsg, WorkerId};
 use embedder_traits::{
-    AnimationState, EmbedderMsg, MediaSessionEvent, TouchEventResult, ViewportDetails,
+    AnimationState, EmbedderMsg, FocusSequenceNumber, MediaSessionEvent, TouchEventResult,
+    ViewportDetails,
 };
 use euclid::default::Size2D as UntypedSize2D;
 use http::{HeaderMap, Method};
@@ -519,8 +520,21 @@ pub enum ScriptToConstellationMessage {
         UntypedSize2D<u64>,
         IpcSender<(IpcSender<CanvasMsg>, CanvasId, ImageKey)>,
     ),
-    /// Notifies the constellation that this frame has received focus.
-    Focus,
+    /// Notifies the constellation that this pipeline is requesting focus.
+    ///
+    /// When this message is sent, the sender pipeline has already its local
+    /// focus state updated. The constellation, after receiving this message,
+    /// will broadcast messages to other pipelines that are affected by this
+    /// focus operation.
+    ///
+    /// The first field contains the browsing context ID of the container
+    /// element if one was focused.
+    ///
+    /// The second field is a sequence number that the constellation should use
+    /// when sending a focus-related message to the sender pipeline next time.
+    Focus(Option<BrowsingContextId>, FocusSequenceNumber),
+    /// Requests the constellation to focus the specified browsing context.
+    FocusRemoteDocument(BrowsingContextId),
     /// Get the top-level browsing context info for a given browsing context.
     GetTopForBrowsingContext(BrowsingContextId, IpcSender<Option<WebViewId>>),
     /// Get the browsing context id of the browsing context in which pipeline is

components/shared/embedder/lib.rs

@@ -14,7 +14,7 @@ pub mod user_content_manager;
 mod webdriver;
 
 use std::ffi::c_void;
-use std::fmt::{Debug, Error, Formatter};
+use std::fmt::{Debug, Display, Error, Formatter};
 use std::path::PathBuf;
 use std::sync::Arc;
 
@@ -784,3 +784,76 @@ pub enum AnimationState {
     /// No animations are active but callbacks are queued
     NoAnimationCallbacksPresent,
 }
+
+/// A sequence number generated by a script thread for its pipelines. The
+/// constellation attaches the target pipeline's last seen `FocusSequenceNumber`
+/// to every focus-related message it sends.
+///
+/// This is used to resolve the inconsistency that occurs due to bidirectional
+/// focus state synchronization and provide eventual consistency. Example:
+///
+/// ```text
+/// script                            constellation
+/// -----------------------------------------------------------------------
+/// send ActivateDocument ----------> receive ActivateDocument
+///                             ,---- send FocusDocument
+///                             |
+/// focus an iframe             |
+/// send Focus -----------------|---> receive Focus
+///                             |     focus the iframe's content document
+/// receive FocusDocument <-----'     send FocusDocument to the content pipeline --> ...
+/// unfocus the iframe
+/// focus the document
+///
+/// Final state:                      Final state:
+///  the iframe is not focused         the iframe is focused
+/// ```
+///
+/// When the above sequence completes, from the script thread's point of view,
+/// the iframe is unfocused, but from the constellation's point of view, the
+/// iframe is still focused.
+///
+/// This inconsistency can be resolved by associating a sequence number to each
+/// message. Whenever a script thread initiates a focus operation, it generates
+/// and sends a brand new sequence number. The constellation attaches the
+/// last-received sequence number to each message it sends. This way, the script
+/// thread can discard out-dated incoming focus messages, and eventually, all
+/// actors converge to the consistent state which is determined based on the
+/// last focus message received by the constellation.
+///
+/// ```text
+/// script                            constellation
+/// -----------------------------------------------------------------------
+/// send ActivateDocument ----------> receive ActivateDocument
+///                             ,---- send FocusDocument (0)
+///                             |
+/// seq_number += 1             |
+/// focus an iframe             |
+/// send Focus (1) -------------|---> receive Focus (1)
+///                             |     focus the iframe's content document
+/// receive FocusDocument (0) <-'     send FocusDocument to the content pipeline --> ...
+/// ignore it because 0 < 1
+///
+/// Final state:                      Final state:
+///  the iframe is focused             the iframe is focused
+/// ```
+#[derive(
+    Clone,
+    Copy,
+    Debug,
+    Default,
+    Deserialize,
+    Eq,
+    Hash,
+    MallocSizeOf,
+    PartialEq,
+    Serialize,
+    PartialOrd,
+)]
+pub struct FocusSequenceNumber(pub u64);
+
+impl Display for FocusSequenceNumber {
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error> {
+        Display::fmt(&self.0, f)
+    }
+}

components/shared/script/lib.rs

@@ -27,8 +27,8 @@ use crossbeam_channel::{RecvTimeoutError, Sender};
 use devtools_traits::ScriptToDevtoolsControlMsg;
 use embedder_traits::user_content_manager::UserContentManager;
 use embedder_traits::{
-    CompositorHitTestResult, InputEvent, MediaSessionActionType, Theme, ViewportDetails,
-    WebDriverScriptCommand,
+    CompositorHitTestResult, FocusSequenceNumber, InputEvent, MediaSessionActionType, Theme,
+    ViewportDetails, WebDriverScriptCommand,
 };
 use euclid::{Rect, Scale, Size2D, UnknownUnit};
 use ipc_channel::ipc::{IpcReceiver, IpcSender};
@@ -191,7 +191,15 @@ pub enum ScriptThreadMessage {
     RemoveHistoryStates(PipelineId, Vec<HistoryStateId>),
     /// Set an iframe to be focused. Used when an element in an iframe gains focus.
     /// PipelineId is for the parent, BrowsingContextId is for the nested browsing context
-    FocusIFrame(PipelineId, BrowsingContextId),
+    FocusIFrame(PipelineId, BrowsingContextId, FocusSequenceNumber),
+    /// Focus the document. Used when the container gains focus.
+    FocusDocument(PipelineId, FocusSequenceNumber),
+    /// Notifies that the document's container (e.g., an iframe) is not included
+    /// in the top-level browsing context's focus chain (not considering system
+    /// focus) anymore.
+    ///
+    /// Obviously, this message is invalid for a top-level document.
+    Unfocus(PipelineId, FocusSequenceNumber),
     /// Passes a webdriver command to the script thread for execution
     WebDriverScriptCommand(PipelineId, WebDriverScriptCommand),
     /// Notifies script thread that all animations are done