Bug 1945988 - Make PresShell::HandleEvent notify editor of mouse button events before handling them r=m_kato,smaug

Chrome dispatches `compositionend` before `mousedown` when there is a
composition and user clicks somewhere.  However, we commit composition when
`EventStateManager::PostHandleEvent` moves/clears focus after `eMouseDown`
dispatching.  Therefore, our `compositionend` event is fired after `mousedown`.

The Chrome's behavior is simpler than us from mouse button event handlers point
of view because it guarantees that there is no composition when handling mouse
button events.  We should follow this behavior for better compatibility and
making the things simpler.

Differential Revision: https://phabricator.services.mozilla.com/D236827

layout/base/PresShell.cpp

@@ -57,6 +57,7 @@
 #include "mozilla/dom/Touch.h"
 #include "mozilla/dom/TouchEvent.h"
 #include "mozilla/dom/UserActivation.h"
+#include "mozilla/EditorBase.h"
 #include "mozilla/ErrorResult.h"
 #include "mozilla/EventDispatcher.h"
 #include "mozilla/EventForwards.h"
@@ -110,6 +111,7 @@
 #include "mozilla/SVGFragmentIdentifier.h"
 #include "mozilla/SVGObserverUtils.h"
 #include "mozilla/Telemetry.h"
+#include "mozilla/TextComposition.h"
 #include "mozilla/TextEvents.h"
 #include "mozilla/TimeStamp.h"
 #include "mozilla/TouchEvents.h"
@@ -7147,6 +7149,53 @@ nsresult PresShell::HandleEvent(nsIFrame* aFrameForPresShell,
     return NS_OK;
   }
 
+  // If there is a composition and we got a pointing device events which may not
+  // impossible to continue the composition, we should notify the editor of the
+  // event before dispatching it.  Then, composition will be commited before
+  // the editor loses focus.  This behavior is compatible with Chrome.
+  // FIXME: Perhaps, we should do same thing before dispatching touch events.
+  switch (aGUIEvent->mMessage) {
+    case eMouseDown:
+    case eMouseUp: {
+      nsPIDOMWindowOuter* const focusedWindow =
+          nsFocusManager::GetFocusedWindowStatic();
+      if (!focusedWindow) {
+        break;
+      }
+      Document* const focusedDocument = focusedWindow->GetExtantDoc();
+      if (!focusedDocument) {
+        break;
+      }
+      nsPresContext* const focusedPresContext =
+          focusedDocument->GetPresContext();
+      if (!focusedPresContext) {
+        break;
+      }
+      const RefPtr<TextComposition> textComposition =
+          IMEStateManager::GetTextCompositionFor(focusedPresContext);
+      if (!textComposition) {
+        break;
+      }
+      // If there is a composition and it's managed by an editor, let's notify
+      // the editor of mouse button event.  The editor commits the composition
+      // unless IME consumes the event.
+      if (RefPtr<EditorBase> editorBase = textComposition->GetEditorBase()) {
+        MOZ_ASSERT(aGUIEvent->AsMouseEvent());
+        editorBase->WillHandleMouseButtonEvent(*aGUIEvent->AsMouseEvent());
+      }
+      // Otherwise, we should commit the orphan composition instead.
+      else if (nsCOMPtr<nsIWidget> widget = textComposition->GetWidget()) {
+        textComposition->RequestToCommit(widget, false);
+      }
+      if (!CanHandleUserInputEvents(aGUIEvent)) {
+        return NS_OK;
+      }
+      break;
+    }
+    default:
+      break;
+  }
+
   if (mPresContext) {
     switch (aGUIEvent->mMessage) {
       case eMouseMove: