Bug 1744573: Maintain a reference to old ComputedStyle struct on LocalAccessibles r=Jamie,emilio

Differential Revision: https://phabricator.services.mozilla.com/D136435

accessible/base/nsAccessibilityService.cpp

@@ -372,6 +372,21 @@ void nsAccessibilityService::NotifyOfPossibleBoundsChange(
   }
 }
 
+void nsAccessibilityService::NotifyOfComputedStyleChange(
+    mozilla::PresShell* aPresShell, nsIContent* aContent) {
+  if (!StaticPrefs::accessibility_cache_enabled_AtStartup()) {
+    return;
+  }
+
+  DocAccessible* document = GetDocAccessible(aPresShell);
+  if (document) {
+    LocalAccessible* accessible = document->GetAccessible(aContent);
+    if (accessible) {
+      accessible->MaybeQueueCacheUpdateForStyleChanges();
+    }
+  }
+}
+
 void nsAccessibilityService::NotifyOfResolutionChange(
     mozilla::PresShell* aPresShell, float aResolution) {
   if (StaticPrefs::accessibility_cache_enabled_AtStartup()) {

accessible/base/nsAccessibilityService.h

@@ -236,6 +236,9 @@ class nsAccessibilityService final : public mozilla::a11y::DocManager,
   void NotifyOfPossibleBoundsChange(mozilla::PresShell* aPresShell,
                                     nsIContent* aContent);
 
+  void NotifyOfComputedStyleChange(mozilla::PresShell* aPresShell,
+                                   nsIContent* aContent);
+
   void NotifyOfResolutionChange(mozilla::PresShell* aPresShell,
                                 float aResolution);
 

accessible/generic/LocalAccessible.cpp

@@ -3306,11 +3306,34 @@ already_AddRefed<AccAttributes> LocalAccessible::BundleFieldsForCache(
         }
       }
     }
+
+    if (nsIFrame* frame = GetFrame()) {
+      // Note our frame's current computed style so we can track style changes
+      // later on.
+      mOldComputedStyle = frame->Style();
+    }
   }
 
   return fields.forget();
 }
 
+void LocalAccessible::MaybeQueueCacheUpdateForStyleChanges() {
+  if (!StaticPrefs::accessibility_cache_enabled_AtStartup()) {
+    return;
+  }
+
+  if (nsIFrame* frame = GetFrame()) {
+    const ComputedStyle* newStyle = frame->Style();
+    MOZ_ASSERT(newStyle != mOldComputedStyle, "New style matches old style!");
+
+    // TODO: comparison on a11y-relevant style attributes
+
+    // TODO: Queue cache update, schedule processing on document
+
+    mOldComputedStyle = newStyle;
+  }
+}
+
 nsAtom* LocalAccessible::TagName() const {
   return mContent && mContent->IsElement() ? mContent->NodeInfo()->NameAtom()
                                            : nullptr;

accessible/generic/LocalAccessible.h

@@ -6,6 +6,7 @@
 #ifndef _LocalAccessible_H_
 #define _LocalAccessible_H_
 
+#include "mozilla/ComputedStyle.h"
 #include "mozilla/a11y/Accessible.h"
 #include "mozilla/a11y/AccTypes.h"
 #include "mozilla/a11y/RelationType.h"
@@ -790,6 +791,8 @@ class LocalAccessible : public nsISupports, public Accessible {
    */
   void SendCache(uint64_t aCacheDomain, CacheUpdateType aUpdate);
 
+  void MaybeQueueCacheUpdateForStyleChanges();
+
   virtual nsAtom* TagName() const override;
 
  protected:
@@ -988,6 +991,21 @@ class LocalAccessible : public nsISupports, public Accessible {
   int32_t mIndexInParent;
   Maybe<nsRect> mBounds;
 
+  /**
+   * Maintain a reference to the ComputedStyle of our frame so we can
+   * send cache updates when style changes are observed.
+   *
+   * This RefPtr is initialised in BundleFieldsForCache to the ComputedStyle
+   * for our initial frame.
+   * When a style change is observed in DidSetComputedStyle we call into
+   * MaybeQueueCacheUpdateForStyleChanges. There, we compare a11y-relevant
+   * properties in mOldComputedStyle with the current ComputedStyle fetched
+   * from GetFrame()->Style(). Finally, we send cache updates for attributes
+   * affected by the style change and update mOldComputedStyle to the style of
+   * our current frame.
+   */
+  RefPtr<const ComputedStyle> mOldComputedStyle;
+
   static const uint8_t kStateFlagsBits = 11;
   static const uint8_t kContextFlagsBits = 3;
 

layout/generic/nsIFrame.cpp

@@ -1179,6 +1179,13 @@ void nsIFrame::MarkNeedsDisplayItemRebuild() {
 // Subclass hook for style post processing
 /* virtual */
 void nsIFrame::DidSetComputedStyle(ComputedStyle* aOldComputedStyle) {
+#ifdef ACCESSIBILITY
+  if (nsAccessibilityService* accService = GetAccService()) {
+    // Notify for all frames, including those that will be reconstructed
+    accService->NotifyOfComputedStyleChange(PresShell(), mContent);
+  }
+#endif
+
   MaybeScheduleReflowSVGNonDisplayText(this);
 
   Document* doc = PresContext()->Document();