Prevent stale frames from being drawn / always ensure up-to-date contents in webview (#3668)

* Prevent stale frames being drawn.

At key points where external changes enter the webview, stale images might get rendered. This ensures that a frame showing current state is always shown.

* Only stage single redraw

* Remove potentially superfluous calls to `self.update()`

* Remove potentially superfluous calls to `self.update()`.

I lost this one during some git troubles.

* Revert unrelated change

The function is supposed to take a boolean telling it whether or not the loading succeeded, which it doesn't as is. However, this is unrelated and works either way so I also reverted it again.

* chore: code cleanup

* cleanup: Remove redundant check for presence of callback

A callback will be used either way for this call, so it can be simplified. The check happens inside the handler.

* Add comment explaining why this change is necessary, referencing the relevant PR.

* Clarify comment to answer the why, not the what.

One can see what is being done, why is probably more important.

CONTRIBUTORS

@@ -203,6 +203,7 @@ hideo aoyama <https://github.com/boukendesho>
 Ross Brown <rbrownwsws@googlemail.com>
 🦙 <github.com/iamllama>
 Lukas Sommer <sommerluk@gmail.com>
+Luca Auer <lolle2000.la@gmail.com>
 Lukas Sommer <sommerluk@users.noreply.github.com>
 Niclas Heinz <nheinz@hpost.net>
 Omar Kohl <omarkohl@posteo.net>

qt/aqt/webview.py

@@ -659,17 +659,17 @@ html {{ {font} }}
         page = self.page()
         assert page is not None
 
-        if cb:
-
         def handler(val: Any) -> None:
             if self._shouldIgnoreWebEvent():
                 print("ignored late js callback", cb)
                 return
+            if cb:
                 cb(val)
 
+            # Without the following, stale frames showing previous or corrupt content get occasionally displayed. (see #3668 for more details)
+            self.update()
+
         page.runJavaScript(js, handler)
-        else:
-            page.runJavaScript(js)
 
     def _queueAction(self, name: str, *args: Any) -> None:
         self._pendingActions.append((name, args))