Bug 1916081 - Make `HTMLEditor::FocusedElementOrDocumentBecomesNotEditable` stop using `aHTMLEditor` at adjusting IME state r=m_kato

`aHTMLEditor` may be `nullptr` and there may have been an `HTMLEditor` before
it's called.  Therefore, it may need to adjust IME state even if `aHTMLEditor`
is `nullptr`.  So, it should adjust IME state from `aDocument` which should be
same as `aHTMLEditor->GetDocument()` if `aHTMLEditor` is not `nullptr`.

Note that if `IMEStateManager` thinks another `nsPresContext` has focus, it does
not touch IME state.  Therefore, it's safe to skip checking the focus state of
the `Document`.

Differential Revision: https://phabricator.services.mozilla.com/D220772

editor/libeditor/HTMLEditor.cpp

@@ -863,16 +863,14 @@ nsresult HTMLEditor::FocusedElementOrDocumentBecomesNotEditable(
   // the editing state change.  Note that if the window of the HTMLEditor has
   // already lost focus, we don't need to do that and we should not touch the
   // other windows.
-  if (aHTMLEditor->OurWindowHasFocus()) {
-    if (RefPtr<nsPresContext> presContext = aHTMLEditor->GetPresContext()) {
-      RefPtr<Element> focusedElement =
-          nsFocusManager::GetFocusedElementStatic();
-      MOZ_ASSERT_IF(focusedElement,
-                    focusedElement->GetPresContext(
+  if (const RefPtr<nsPresContext> presContext = aDocument.GetPresContext()) {
+    const RefPtr<Element> focusedElementInDocument =
+        Element::FromNodeOrNull(aDocument.GetUnretargetedFocusedContent());
+    MOZ_ASSERT_IF(focusedElementInDocument,
+                  focusedElementInDocument->GetPresContext(
                       Element::PresContextFor::eForComposedDoc));
     IMEStateManager::MaybeOnEditableStateDisabled(*presContext,
-                                                    focusedElement);
-    }
+                                                  focusedElementInDocument);
   }
 
   return rv;