libservo: Allow the embedder to activate accessibility by delan · Pull Request #42336 · servo/servo

delan · 2026-02-04T15:39:51Z

this patch adds a Servo::set_accessibility_active() method that embedders can use to tell Servo to start building and sending accessibility trees to the platform, as long as the pref is enabled (#42333). doing so sets a global flag in the constellation, which is then propagated to the layout of all existing and future pipelines.

Testing: none yet, no functional change
Fixes: part of #4344

delan · 2026-02-06T10:43:03Z

+    fn set_accessibility_active(&mut self, active: bool) {
+        self.accessibility_active = active;
+        for browsing_context in self.browsing_contexts.values_mut() {
+            if let Some(pipeline) = self.pipelines.get(&browsing_context.pipeline_id) {
+                let _ = pipeline
+                    .event_loop
+                    .send(ScriptThreadMessage::SetAccessibilityActive(
+                        browsing_context.webview_id,
+                        pipeline.id,
+                        active,
+                    ));
+            }
+        }
+    }


@mrobinson said:

Is the idea that this will be enabled or disabled per-WebView in the future? If so, the external-facing API should likely be on WebView in the API. If not (and I think that's a perfectly fine decision), then this code should probably iterate though each EventLoop by iterating through Constellation::event_loops() here.

If you take the second approach, then in ScriptThread (there is one of these per-EventLoop), iterate through all Documents and enable / disable accessibility for every Layout.

@mrobinson Does the latest change do what you had in mind?

Yes! This is much closer. I offered a suggestion for a different approach, but maybe that isn't necessary here. I think it could come in handy when you want to add support disabling accessibility though.

mrobinson · 2026-02-13T17:52:32Z

+            let _ = pipeline
+                .event_loop
+                .send(ScriptThreadMessage::SetAccessibilityActive(true));
+        }


Another option here is to store whether or not accessibility is enabled in each ScriptThread and then seed every new Layout with that information. That would avoid having the possibility of any "in between" time where accessibility seems to be disabled -- maybe a layout happens without it event.

This would require modifying theEventLoop creation code in components/constellation/event_loop.rs. For instance it could be a member of InitialScriptState which is used to initialize the state of a new ScriptThread (one per EventLoop). Then in each ScriptThread you would store the state and ensure that new Layouts have a11y enabled.

I think this is necessary, yeah. So now we have accessibility_active in:

Constellation, so that new ScriptThreads have the right configuration

InitialScriptState, to pass the value through from Constellation to each new ScriptThread

ScriptThread, so that new LayoutThreads have the right configuration

LayoutConfig, to pass the value through from ScriptThread to each new LayoutThread

I added a temporary flag in LayoutThread just to have a place for the config to "land" until we add the AccessibilityTree class.

Does this look right to you? Is there somewhere we're storing the flag unnecessarily now? Admittedly I'm still getting my head around the relationships between each of these objects.

mrobinson · 2026-02-13T17:57:08Z

+    fn set_accessibility_active(&mut self, active: bool) {
+        self.accessibility_active = active;
+        for browsing_context in self.browsing_contexts.values_mut() {
+            if let Some(pipeline) = self.pipelines.get(&browsing_context.pipeline_id) {
+                let _ = pipeline
+                    .event_loop
+                    .send(ScriptThreadMessage::SetAccessibilityActive(
+                        browsing_context.webview_id,
+                        pipeline.id,
+                        active,
+                    ));
+            }
+        }
+    }


Yes! This is much closer. I offered a suggestion for a different approach, but maybe that isn't necessary here. I think it could come in handy when you want to add support disabling accessibility though.

mrobinson

Sorry for the delay in reviewing. Great stuff! Just a couple tiny nits, but feel free to land this when addressed:

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

Now, the constellation loops over its event_loops, and each script_thread loops over its documents. Signed-off-by: Alice Boxhall <alice@igalia.com>

Also adds accessibility_active fields in InitialScriptState and LayoutConfig, so ScriptThread and LayoutThread can be initialized with the correct accessibility_active state. Signed-off-by: Alice Boxhall <alice@igalia.com>

Signed-off-by: Alice Boxhall <alice@igalia.com>

Signed-off-by: delan azabani <dazabani@igalia.com>

in #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 #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 #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>

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>

delan force-pushed the set-accessibility-enabled branch from f5bfb00 to 56054ff Compare February 4, 2026 15:56

alice reviewed Feb 5, 2026

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Comment thread components/constellation/constellation.rs Outdated

Comment thread components/shared/embedder/lib.rs Outdated

delan force-pushed the set-accessibility-enabled branch from 56054ff to 4e85ef3 Compare February 5, 2026 12:06

delan changed the title ~~libservo: Allow the embedder to enable accessibility~~ libservo: Allow the embedder to activate accessibility Feb 5, 2026

delan force-pushed the set-accessibility-enabled branch 3 times, most recently from 1ed4818 to 435e8ce Compare February 5, 2026 12:19

delan commented Feb 5, 2026

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Comment thread components/script/script_thread.rs Outdated

delan marked this pull request as ready for review February 5, 2026 12:22

delan requested review from Loirooriol, gterzian and mrobinson as code owners February 5, 2026 12:22

servo-highfive added the S-awaiting-review There is new code that needs to be reviewed. label Feb 5, 2026

alice mentioned this pull request Feb 5, 2026

config: Add a pref for the experimental accessibility code #42333

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delan commented Feb 6, 2026

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alice force-pushed the set-accessibility-enabled branch 2 times, most recently from 6656b26 to 34fc937 Compare February 11, 2026 14:37

mrobinson reviewed Feb 13, 2026

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alice force-pushed the set-accessibility-enabled branch from b366aec to 06eb1a1 Compare February 17, 2026 10:24

delan requested a review from mrobinson February 19, 2026 10:11

alice force-pushed the set-accessibility-enabled branch from 06eb1a1 to 9db1350 Compare February 19, 2026 10:34

mrobinson approved these changes Feb 19, 2026

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Comment thread components/constellation/constellation.rs

Comment thread components/script/script_thread.rs Outdated

Comment thread components/script/script_thread.rs Outdated

servo-highfive added S-awaiting-review There is new code that needs to be reviewed. and removed S-awaiting-review There is new code that needs to be reviewed. labels Feb 19, 2026

delan force-pushed the set-accessibility-enabled branch from 2e6959d to 24d67f1 Compare February 20, 2026 05:15

delan and others added 5 commits February 20, 2026 13:16

libservo: Allow the embedder to enable accessibility

323b5d0

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

Re-work SetAccessibilityEnabled to take only a boolean.

dd34492

Now, the constellation loops over its event_loops, and each script_thread loops over its documents. Signed-off-by: Alice Boxhall <alice@igalia.com>

Track accessibility_active in ScriptThread as well.

227edc7

Also adds accessibility_active fields in InitialScriptState and LayoutConfig, so ScriptThread and LayoutThread can be initialized with the correct accessibility_active state. Signed-off-by: Alice Boxhall <alice@igalia.com>

Check accessibility_enabled pref before setting accessibility active

8513a17

Signed-off-by: Alice Boxhall <alice@igalia.com>

Return early when accessibility state would not change

6410f3e

Signed-off-by: delan azabani <dazabani@igalia.com>

delan added 2 commits February 20, 2026 13:16

Inline variables only used once

e99938b

Signed-off-by: delan azabani <dazabani@igalia.com>

Return early in ScriptThread too when activation would not change

2ccb910

Signed-off-by: delan azabani <dazabani@igalia.com>

delan force-pushed the set-accessibility-enabled branch from 24d67f1 to 2ccb910 Compare February 20, 2026 05:16

delan enabled auto-merge February 20, 2026 05:16

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servo-highfive added the S-awaiting-merge The PR is in the process of compiling and running tests on the automated CI. label Feb 20, 2026

github-merge-queue Bot removed this pull request from the merge queue due to failed status checks Feb 20, 2026

servo-highfive added S-tests-failed The changes caused existing tests to fail. and removed S-awaiting-merge The PR is in the process of compiling and running tests on the automated CI. labels Feb 20, 2026

Fix compile error under MSRV

35526cd

Signed-off-by: delan azabani <dazabani@igalia.com>

servo-highfive removed the S-tests-failed The changes caused existing tests to fail. label Feb 20, 2026

delan enabled auto-merge February 20, 2026 05:48

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Merged via the queue into servo:main with commit c07682f Feb 20, 2026
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delan deleted the set-accessibility-enabled branch February 20, 2026 06:47

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delan mentioned this pull request Mar 5, 2026

libservo: Per-webview accessibility activation #43029

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libservo: Allow the embedder to activate accessibility#42336

libservo: Allow the embedder to activate accessibility#42336
delan merged 8 commits intoservo:mainfrom
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