fix stuck webview issue

A race condition could cause domDone to get stuck on false, causing
any further bridge messages and setHtml() calls to be ignored. Fix
the issue by clearing pending actions and resetting domDone when
setting HTML.

aqt/webview.py

@@ -170,6 +170,9 @@ class AnkiWebView(QWebEngineView):
         pass
 
     def setHtml(self, html):
+        # discard any previous pending actions
+        self._pendingActions = []
+        self._domDone = True
         self._queueAction("setHtml", html)
 
     def _setHtml(self, html):