From 419456eacd2ed043b28b119dce6246dd7ee282c5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?H=C3=A5kon=20J=20D=20Johnsen?= <hakon.j.d.johnsen@ntnu.no>
Date: Fri, 29 Nov 2019 19:46:46 +0100
Subject: [PATCH] Fix race condition in rendering of preview window

Previously, if repeated calls to _renderPreview() were throttled by
the throttling mechanism, only the most recent value of
cardChanged would take effect, even if the card had changed
in previous _renderPreview() calls.

After this patch, if any of the throttled _renderPreview() calls
were called with cardChanged=True, the final _renderScheduledPreview()
will happen with cardChanged=True
---
 aqt/browser.py | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/aqt/browser.py b/aqt/browser.py
index 04c1136fa..04dffa4ef 100644
--- a/aqt/browser.py
+++ b/aqt/browser.py
@@ -1259,6 +1259,7 @@ where id in %s""" % ids2str(sf))
     _previewTimer = None
     _lastPreviewRender = 0
     _lastPreviewState = None
+    _previewCardChanged = False
 
     def onTogglePreview(self):
         if self._previewWindow:
@@ -1367,20 +1368,23 @@ where id in %s""" % ids2str(sf))
 
     def _renderPreview(self, cardChanged=False):
         self._cancelPreviewTimer()
+        # Keep track of whether _renderPreview() has ever been called
+        # with cardChanged=True since the last successful render
+        self._previewCardChanged |= cardChanged
         # avoid rendering in quick succession
         elapMS = int((time.time() - self._lastPreviewRender)*1000)
         if elapMS < 500:
             self._previewTimer = self.mw.progress.timer(
-                500-elapMS, lambda: self._renderScheduledPreview(cardChanged), False)
+                500-elapMS, lambda: self._renderScheduledPreview(), False)
         else:
-            self._renderScheduledPreview(cardChanged)
+            self._renderScheduledPreview()
 
     def _cancelPreviewTimer(self):
         if self._previewTimer:
             self._previewTimer.stop()
             self._previewTimer = None
 
-    def _renderScheduledPreview(self, cardChanged=False):
+    def _renderScheduledPreview(self):
         self._cancelPreviewTimer()
         self._lastPreviewRender = time.time()
 
@@ -1395,7 +1399,7 @@ where id in %s""" % ids2str(sf))
         else:
             if self._previewBothSides:
                 self._previewState = "answer"
-            elif cardChanged:
+            elif self._previewCardChanged:
                 self._previewState = "question"
 
             currentState = self._previewStateAndMod()
@@ -1433,6 +1437,7 @@ where id in %s""" % ids2str(sf))
         self._updatePreviewButtons()
         self._previewWeb.eval(
             "{}({},'{}');".format(func, json.dumps(txt), bodyclass))
+        self._previewCardChanged = False
 
     def _onPreviewShowBothSides(self, toggle):
         self._previewBothSides = toggle