Bug 1896816: Fail gracefully if a naughty client tries to QueryInterface on an MsaaAccessible from the wrong thread. r=eeejay

JAWS apparently does this sometimes.
In particular, this was causing a crash when we tried to check the UIA pref because that pref can only be accessed from the main thread.

Differential Revision: https://phabricator.services.mozilla.com/D210417

accessible/windows/msaa/MsaaAccessible.cpp

@@ -508,6 +508,13 @@ MsaaAccessible::QueryInterface(REFIID iid, void** ppv) {
     return E_NOINTERFACE;
   }
 
+  if (NS_WARN_IF(!NS_IsMainThread())) {
+    // Bug 1896816: JAWS sometimes traverses into Gecko UI from a file dialog
+    // thread. It shouldn't do that, but let's fail gracefully instead of
+    // crashing.
+    return RPC_E_WRONG_THREAD;
+  }
+
   // These interfaces are always available. We can support querying to them
   // even if the Accessible is dead.
   if (IID_IUnknown == iid) {