Bug 1629347 - Make `TSFTextStore::SetInputContext()` emulate focus when the menu keyboard event listener is uninstalled r=m_kato

Currently, `IMEStateManager` sets `InputContext` to disable IME when the
menu keyboard event listener is installed, but does not destroy
`IMEContentObserver` which may be in a remote process.  And similarly,
it setns `InputContext` to enable IME when the menu keyboard event listener
is uninstalled, but does not make `IMEContentObserver` which may be in a remote
process send `NOTIFY_IME_OF_FOCUS` again.  Therefore, from point of view of
native IME handlers in widget, `NOTIFY_IME_OF_FOCUS` won't be sent in this
case, but `TSFTextStore::SetInputContext()` expects that it'll be followed
by `NOTIFY_IME_OF_FOCUS` unless it's called for `FOCUS_NOT_CHANGED`.
Therefore, `sEnabledTextStore` is `nullptr` when this bug occurs.

For fixing this bug **quickly**, this patch makes
`TSFTextStore::SetInputContext()` treat  `MENU_LOST_PSEUDO_FOCUS` as
`FOCUS_NOT_CHANGED` to create `TSFTextStore` instance if new state is
"enabled".

Ideally, we should redesign `IMEStateManager` because while menu keyboard
event listener has pseudo focus, web apps may change focus or create something
complicated cases.

Unfortunately, we still cannot write automated tests for `TSFTextStore`
due to bug 1322744.

Differential Revision: https://phabricator.services.mozilla.com/D93469

widget/windows/TSFTextStore.cpp

@@ -6589,28 +6589,41 @@ void TSFTextStore::SetInputContext(nsWindowBase* aWidget,
            sEnabledTextStore ? sEnabledTextStore->mWidget.get() : nullptr,
            GetBoolName(ThinksHavingFocus())));
 
-  // When this is called when the widget is created, there is nothing to do.
-  if (aAction.mFocusChange == InputContextAction::WIDGET_CREATED) {
-    return;
-  }
-
-  NS_ENSURE_TRUE_VOID(IsInTSFMode());
-
-  if (aAction.mFocusChange != InputContextAction::FOCUS_NOT_CHANGED) {
-    if (sEnabledTextStore) {
-      RefPtr<TSFTextStore> textStore(sEnabledTextStore);
-      textStore->SetInputScope(aContext.mHTMLInputType,
-                               aContext.mHTMLInputInputmode,
-                               aContext.mInPrivateBrowsing);
-    }
-    return;
+  switch (aAction.mFocusChange) {
+    case InputContextAction::WIDGET_CREATED:
+      // If this is called when the widget is created, there is nothing to do.
+      return;
+    case InputContextAction::FOCUS_NOT_CHANGED:
+    case InputContextAction::MENU_LOST_PSEUDO_FOCUS:
+      if (NS_WARN_IF(!IsInTSFMode())) {
+        return;
+      }
+      // In these cases, `NOTIFY_IME_OF_FOCUS` won't be sent.  Therefore,
+      // we need to reset text store for new state right now.
+      break;
+    default:
+      NS_WARNING_ASSERTION(IsInTSFMode(),
+                           "Why is this called when TSF is disabled?");
+      if (sEnabledTextStore) {
+        RefPtr<TSFTextStore> textStore(sEnabledTextStore);
+        textStore->SetInputScope(aContext.mHTMLInputType,
+                                 aContext.mHTMLInputInputmode,
+                                 aContext.mInPrivateBrowsing);
+      }
+      return;
   }
 
   // If focus isn't actually changed but the enabled state is changed,
   // emulate the focus move.
   if (!ThinksHavingFocus() && aContext.mIMEState.IsEditable()) {
+    MOZ_LOG(sTextStoreLog, LogLevel::Debug,
+            ("  TSFTextStore::SetInputContent() emulates focus for IME "
+             "state change"));
     OnFocusChange(true, aWidget, aContext);
   } else if (ThinksHavingFocus() && !aContext.mIMEState.IsEditable()) {
+    MOZ_LOG(sTextStoreLog, LogLevel::Debug,
+            ("  TSFTextStore::SetInputContent() emulates blur for IME "
+             "state change"));
     OnFocusChange(false, aWidget, aContext);
   }
 }