Bug 1820448 - Make `RangeUpdater::SelAdjSplitNode` handle insertion better r=m_kato

The root cause of this bug is, we tried to compute new offset with offset at
the new node.  However, as explained in the inline comments, it should compute
the offset with the right node offset in the new mode.  Therefore, it needs
to handle it by itself instead of just calling `SelAdjInsertNode`.

Differential Revision: https://phabricator.services.mozilla.com/D171965

editor/libeditor/SelectionState.cpp

@@ -5,8 +5,9 @@
 
 #include "SelectionState.h"
 
-#include "AutoRangeArray.h"          // for AutoRangeArray
-#include "EditorUtils.h"             // for EditorUtils, AutoRangeArray
+#include "AutoRangeArray.h"  // for AutoRangeArray
+#include "EditorUtils.h"     // for EditorUtils, AutoRangeArray
+#include "ErrorList.h"
 #include "JoinSplitNodeDirection.h"  // for JoinNodesDirection, SplitNodeDirection
 
 #include "mozilla/Assertions.h"    // for MOZ_ASSERT, etc.
@@ -311,18 +312,41 @@ nsresult RangeUpdater::SelAdjSplitNode(nsIContent& aOriginalContent,
   }
 
   EditorRawDOMPoint atNewNode(&aNewContent);
-  nsresult rv = SelAdjInsertNode(atNewNode);
-  if (NS_FAILED(rv)) {
-    NS_WARNING("RangeUpdater::SelAdjInsertNode() failed");
-    return rv;
+  if (NS_WARN_IF(!atNewNode.IsSetAndValid())) {
+    return NS_ERROR_EDITOR_UNEXPECTED_DOM_TREE;
   }
 
-  // If point is in the range which are moved from aOriginalContent to
-  // aNewContent, we need to change its container to aNewContent and may need to
-  // adjust the offset. If point is in the range which are not moved from
-  // aOriginalContent, we may need to adjust the offset.
   auto AdjustDOMPoint = [&](nsCOMPtr<nsINode>& aContainer,
                             uint32_t& aOffset) -> void {
+    if (aContainer == atNewNode.GetContainer()) {
+      if (aSplitNodeDirection == SplitNodeDirection::LeftNodeIsNewOne) {
+        // When we create a left node, we insert it before the right node.
+        // In this case,
+        // - `{}<right/>` should become `{}<left/><right/>` (0 -> 0)
+        // - `<right/>{}` should become `<left/><right/>{}` (1 -> 2)
+        // - `{<right/>}` should become `{<left/><right/>}` (0 -> 0, 1 -> 2}
+        // Therefore, we need to increate the offset only when the offset is
+        // larger than the offset at the left node.
+        if (aOffset > atNewNode.Offset()) {
+          aOffset++;
+        }
+      } else {
+        // When we create a right node, we insert it after the left node.
+        // In this case,
+        // - `{}<left/>` should become `{}<left/><right/>` (0 -> 0)
+        // - `<left/>{}` should become `<left/><right/>{}` (1 -> 2)
+        // - `{<left/>}` should become `{<left/><right/>}` (0 -> 0, 1 -> 2}
+        // Therefore, we need to increate the offset only when the offset equals
+        // or is larger than the offset at the right node.
+        if (aOffset >= atNewNode.Offset()) {
+          aOffset++;
+        }
+      }
+    }
+    // If point is in the range which are moved from aOriginalContent to
+    // aNewContent, we need to change its container to aNewContent and may need
+    // to adjust the offset. If point is in the range which are not moved from
+    // aOriginalContent, we may need to adjust the offset.
     if (aContainer != &aOriginalContent) {
       return;
     }