Bug 1583766 - Consider `IMEContentObserver` to send selection change notification when its sender sends a text change notification r=m_kato

The previous change is wrong because the sender shouldn't send selection change
notification if `IMEContentObserver` does not receive text change notifications
nor selection change notifications.

This patch reverts the change in the if block of previous change and instead
sets `IMEContentObserver::mNeedsToNotifyIMEOfSelectionChange` to `true` if
the sender sends a text change notification.  (Note that even if there is
another text change with a mutation event listener, we need to send selection
change notification later so that this must be the right approach.)

Differential Revision: https://phabricator.services.mozilla.com/D47564

--HG--
extra : moz-landing-system : lando

dom/events/IMEContentObserver.cpp

@@ -1680,24 +1680,27 @@ IMEContentObserver::IMENotificationSender::Run() {
   if (observer->mNeedsToNotifyIMEOfTextChange) {
     observer->mNeedsToNotifyIMEOfTextChange = false;
     SendTextChange();
+    // Even if the observer hasn't received selection change, let's try to send
+    // selection change notification to IME because selection start offset may
+    // be changed if the previous contents of selection start are changed.  For
+    // example, when previous `<p>` element of another `<p>` element which
+    // contains caret is removed by a DOM mutation, selection change event
+    // won't be fired, but selection start offset should be decreased by the
+    // length of removed `<p>` element.
+    observer->mNeedsToNotifyIMEOfSelectionChange = true;
   }
 
   // If a text change notification causes another text change again, we should
   // notify IME of that before sending a selection change notification.
-  // Otherwise, even if the observer hasn't received selection change, let's
-  // try to send selection change notification to IME because selection
-  // start offset may be changed if the previous contents of selection start
-  // are changed.  For example, when previous `<p>` element of another `<p>`
-  // element which contains caret is removed by a DOM mutation, selection
-  // change event won't be fired, but selection start offset should be
-  // decreased by the length of removed `<p>` element.
   if (!observer->mNeedsToNotifyIMEOfTextChange) {
     // Be aware, PuppetWidget depends on the order of this. A selection change
     // notification should not be sent before a text change notification because
     // PuppetWidget shouldn't query new text content every selection change.
+    if (observer->mNeedsToNotifyIMEOfSelectionChange) {
       observer->mNeedsToNotifyIMEOfSelectionChange = false;
       SendSelectionChange();
     }
+  }
 
   // If a text change notification causes another text change again or a
   // selection change notification causes either a text change or another