diff --git a/dom/html/HTMLInputElement.cpp b/dom/html/HTMLInputElement.cpp
index b69e73bfabdf..eb234c061bdf 100644
--- a/dom/html/HTMLInputElement.cpp
+++ b/dom/html/HTMLInputElement.cpp
@@ -6539,22 +6539,8 @@ HTMLInputElement::SetSelectionDirection(const nsAString& aDirection, ErrorResult
   }
 
   nsTextEditorState* state = GetEditorState();
-  if (state && state->IsSelectionCached()) {
-    nsITextControlFrame::SelectionDirection dir = nsITextControlFrame::eNone;
-    if (aDirection.EqualsLiteral("forward")) {
-      dir = nsITextControlFrame::eForward;
-    } else if (aDirection.EqualsLiteral("backward")) {
-      dir = nsITextControlFrame::eBackward;
-    }
-    state->GetSelectionProperties().SetDirection(dir);
-    return;
-  }
-
-  int32_t start, end;
-  GetSelectionRange(&start, &end, aRv);
-  if (!aRv.Failed()) {
-    aRv = SetSelectionRange(start, end, aDirection);
-  }
+  MOZ_ASSERT(state, "SupportsTextSelection came back true!");
+  state->SetSelectionDirection(aDirection, aRv);
 }
 
 NS_IMETHODIMP
diff --git a/dom/html/HTMLTextAreaElement.cpp b/dom/html/HTMLTextAreaElement.cpp
index 8584c5355139..c00d3507f63d 100644
--- a/dom/html/HTMLTextAreaElement.cpp
+++ b/dom/html/HTMLTextAreaElement.cpp
@@ -765,27 +765,7 @@ void
 HTMLTextAreaElement::SetSelectionDirection(const nsAString& aDirection,
                                            ErrorResult& aError)
 {
-  if (mState.IsSelectionCached()) {
-    nsITextControlFrame::SelectionDirection dir = nsITextControlFrame::eNone;
-    if (aDirection.EqualsLiteral("forward")) {
-      dir = nsITextControlFrame::eForward;
-    } else if (aDirection.EqualsLiteral("backward")) {
-      dir = nsITextControlFrame::eBackward;
-    }
-    mState.GetSelectionProperties().SetDirection(dir);
-    return;
-  }
-
-  int32_t start, end;
-  GetSelectionRange(&start, &end, aError);
-  if (aError.Failed()) {
-    return;
-  }
-
-  nsresult rv = SetSelectionRange(start, end, aDirection);
-  if (NS_FAILED(rv)) {
-    aError.Throw(rv);
-  }
+  mState.SetSelectionDirection(aDirection, aError);
 }
 
 NS_IMETHODIMP
diff --git a/dom/html/nsTextEditorState.cpp b/dom/html/nsTextEditorState.cpp
index 8bad91001b57..6bbd2e0c12bd 100644
--- a/dom/html/nsTextEditorState.cpp
+++ b/dom/html/nsTextEditorState.cpp
@@ -1762,6 +1762,38 @@ nsTextEditorState::SetSelectionEnd(const mozilla::dom::Nullable<uint32_t>& aEnd,
   SetSelectionRange(start, end, dir, aRv);
 }
 
+static nsITextControlFrame::SelectionDirection
+DirectionStringToSelectionDirection(const nsAString& aDirection)
+{
+  if (aDirection.EqualsLiteral("backward")) {
+    return nsITextControlFrame::eBackward;
+  }
+
+  // We don't support directionless selections.
+  return nsITextControlFrame::eForward;
+}
+
+void
+nsTextEditorState::SetSelectionDirection(const nsAString& aDirection,
+                                         ErrorResult& aRv)
+{
+  nsITextControlFrame::SelectionDirection dir =
+    DirectionStringToSelectionDirection(aDirection);
+
+  if (IsSelectionCached()) {
+    GetSelectionProperties().SetDirection(dir);
+    return;
+  }
+
+  int32_t start, end;
+  GetSelectionRange(&start, &end, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  SetSelectionRange(start, end, dir, aRv);
+}
+
 HTMLInputElement*
 nsTextEditorState::GetParentNumberControl(nsFrame* aFrame) const
 {
diff --git a/dom/html/nsTextEditorState.h b/dom/html/nsTextEditorState.h
index dbbc8494f945..0dd36458ff16 100644
--- a/dom/html/nsTextEditorState.h
+++ b/dom/html/nsTextEditorState.h
@@ -310,6 +310,12 @@ public:
   void SetSelectionEnd(const mozilla::dom::Nullable<uint32_t>& aEnd,
                        mozilla::ErrorResult& aRv);
 
+  // Set the selection direction.  This basically implements the
+  // https://html.spec.whatwg.org/multipage/forms.html#dom-textarea/input-selectiondirection
+  // setter.
+  void SetSelectionDirection(const nsAString& aDirection,
+                             mozilla::ErrorResult& aRv);
+
   void UpdateEditableState(bool aNotify) {
     if (mRootNode) {
       mRootNode->UpdateEditableState(aNotify);