Expose an embedded object character only for nodes visible to platform APIs

We don't need to use an object replacement character in IAccessible2, UI Automation and Linux ATK
if the node whose text is replacing is not visible to those APIs,
i.e. if the node lies outside the part of the accessibility tree that could be
accessed by platform APIs.
Object replacement characters are only meaningful when they appear in the accessible text for IAccessible2 and ATK,
and only when they are used as a word or character boundary for UI Automation.
Therefore, any object replacement character that cannot be accessed either via IAccessibleText, UIATextRangeProvider,, ATKText, etc.
is unnecessary. We currently don't expose such characters in IA2 hypertext, for example.
It can also make it impossible to compute the equivalent ancestor AXPosition on a text field for
a text position that is pointing to an inline text box inside that field.

This patch improves and generalizes the fix in
https://chromium-review.googlesource.com/c/chromium/src/+/2473065

AX-Relnotes: n/a.

R=dmazzoni@chromium.org, kschmidt@microsoft.com, vicfei@microsoft.com

Bug: 1141171
Change-Id: Ia6699a841dfca59558ed1188de04a06e5b13e66c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2504504Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Victor Fei <vicfei@microsoft.com>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Auto-Submit: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828959}

ui/accessibility/ax_node_position.cc

@@ -257,18 +257,24 @@ bool AXNodePosition::IsEmbeddedObjectInParent() const {
     case AXEmbeddedObjectBehavior::kSuppressCharacter:
       return false;
     case AXEmbeddedObjectBehavior::kExposeCharacter:
-      // We don't need to expose an "embedded object character" for textual
-      // nodes as well as nodes that are invisible to platform APIs, AKA nodes
-      // that are descendants of platform leaves. In the former case, textual
-      // nodes are represented by their actual text in the text of their parent
-      // nodes, in order to maintain compatibility with how Firefox exposes text
-      // in IAccessibleText. For the latter case, an example of a platform leaf
-      // is a plain text field.
+      // We expose an "object replacement character" for all nodes except
+      // textual nodes as well as nodes that are invisible to platform APIs, AKA
+      // nodes that are descendants of platform leaves. In the former case,
+      // textual nodes are represented by their actual text in the text of their
+      // parent nodes, in order to maintain compatibility with how Firefox
+      // exposes text in IAccessibleText. For the latter case, an example of a
+      // platform leaf is a plain text field because all of the accessibility
+      // subtree inside the text field is not visible to platform APIs.
       //
-      // On the other hand, we do need to expose an "embedded object character"
-      // for empty objects, such as an empty text field, for navigational
-      // purposes. This is because such objects need to act as a word and
-      // character boundary.
+      // Please note that for navigational purposes, we need to expose an
+      // "object replacement character" in empty controls, such as in an empty
+      // text field. The presence or the absence of accessible content inside a
+      // control might alter whether an "object replacement character" would be
+      // exposed in that control, in contrast to ordinary text such as in the
+      // case of a non-empty simple text field which should only have textual
+      // nodes inside it. This is because empty controls need to act as a word
+      // and character boundary. See
+      // "AXPosition::IsEmptyObjectReplacedByCharacter()" for more information.
       return !IsNullPosition() && !GetAnchor()->IsText() &&
              !GetAnchor()->IsChildOfLeaf();
   }

ui/accessibility/ax_position.h

@@ -3399,10 +3399,18 @@ class AXPosition {
       text_offset_ = max_text_offset;
   }
 
-  // Returns true if this position is on an empty object node that needs to
-  // be represented by an empty object replacement character. It does when the
-  // node is a collapsed menu list popup button or has no unignored child and is
-  // not a text object. This feature is only enabled on some platforms.
+  // Returns true if this position is on an empty control that needs to be
+  // represented by an "object replacement character". This feature is only
+  // enabled on some platforms.
+  //
+  // This is purely for navigational purposes. We need to expose an "object
+  // replacement character" in empty controls, such as in an empty text field or
+  // a collapsed popup menu. The presence or the absence of accessible content
+  // inside a control might alter whether an "object replacement character"
+  // would be exposed in that control, in contrast to ordinary text such as in
+  // the case of a non-empty simple text field which should only have textual
+  // nodes inside it. This is because empty controls need to act as a word and
+  // character boundary.
   bool IsEmptyObjectReplacedByCharacter() const {
     if (g_ax_embedded_object_behavior ==
             AXEmbeddedObjectBehavior::kSuppressCharacter ||