Bug 1819802 part 1: When getting cached a11y character bounds, return…

… a single rect rather than an array of rects. r=morgan

When hit testing, we retrieve the bounds for every character in a text leaf in a separate call.
Because we need to convert from an array of flat ints to rects, this previously meant we built the entire array for every character.
For a large text leaf, building this rect array is relatively expensive.
We only need a single rect at a time.
Therefore, RemoteAccessibleBase now returns the rect for a single requested character instead of building an array of rects.

Differential Revision: https://phabricator.services.mozilla.com/D171419

accessible/base/CacheConstants.h

@@ -83,6 +83,10 @@ static constexpr RelationData kRelationTypeAtoms[] = {
      RelationType::FLOWS_FROM},
 };
 
+// The count of numbers needed to serialize an nsRect. This is used when
+// flattening character rects into an array of ints.
+constexpr int32_t kNumbersInRect = 4;
+
 }  // namespace a11y
 }  // namespace mozilla
 

accessible/base/TextLeafRange.cpp

@@ -1668,13 +1668,9 @@ LayoutDeviceIntRect TextLeafPoint::CharBounds() {
   }
 
   RemoteAccessible* remote = mAcc->AsRemote();
-  if (Maybe<nsTArray<nsRect>> charBounds = remote->GetCachedCharData()) {
-    if (mOffset < static_cast<int32_t>(charBounds->Length())) {
-      return remote->BoundsWithOffset(Some(charBounds->ElementAt(mOffset)));
-    }
-    // It is valid for a client to call this with an offset 1 after the last
-    // character because of the insertion point at the end of text boxes.
-    MOZ_ASSERT(mOffset == static_cast<int32_t>(charBounds->Length()));
+  nsRect charBounds = remote->GetCachedCharRect(mOffset);
+  if (!charBounds.IsEmpty()) {
+    return remote->BoundsWithOffset(Some(charBounds));
   }
 
   return LayoutDeviceIntRect();

accessible/ipc/RemoteAccessibleBase.cpp

@@ -1064,25 +1064,27 @@ RemoteAccessibleBase<Derived>::GetCachedTextLines() {
 }
 
 template <class Derived>
-Maybe<nsTArray<nsRect>> RemoteAccessibleBase<Derived>::GetCachedCharData() {
+nsRect RemoteAccessibleBase<Derived>::GetCachedCharRect(int32_t aOffset) {
   MOZ_ASSERT(IsText());
   if (!mCachedFields) {
-    return Nothing();
+    return nsRect();
   }
 
   if (Maybe<const nsTArray<int32_t>&> maybeCharData =
           mCachedFields->GetAttribute<nsTArray<int32_t>>(
               nsGkAtoms::characterData)) {
     const nsTArray<int32_t>& charData = *maybeCharData;
-    nsTArray<nsRect> rects;
-    for (int i = 0; i < static_cast<int32_t>(charData.Length()); i += 4) {
-      nsRect r(charData[i], charData[i + 1], charData[i + 2], charData[i + 3]);
-      rects.AppendElement(r);
+    const int32_t index = aOffset * kNumbersInRect;
+    if (index < static_cast<int32_t>(charData.Length())) {
+      return nsRect(charData[index], charData[index + 1], charData[index + 2],
+                    charData[index + 3]);
     }
-    return Some(std::move(rects));
+    // It is valid for a client to call this with an offset 1 after the last
+    // character because of the insertion point at the end of text boxes.
+    MOZ_ASSERT(index == static_cast<int32_t>(charData.Length()));
   }
 
-  return Nothing();
+  return nsRect();
 }
 
 template <class Derived>

accessible/ipc/RemoteAccessibleBase.h

@@ -364,7 +364,7 @@ class RemoteAccessibleBase : public Accessible, public HyperTextAccessibleBase {
 
   uint32_t GetCachedTextLength();
   Maybe<const nsTArray<int32_t>&> GetCachedTextLines();
-  Maybe<nsTArray<nsRect>> GetCachedCharData();
+  nsRect GetCachedCharRect(int32_t aOffset);
   RefPtr<const AccAttributes> GetCachedTextAttributes();
   RefPtr<const AccAttributes> GetCachedARIAAttributes() const;