Revise comment about random but regular test suite crashes.

securedrop_client/gui/widgets.py

@@ -1990,10 +1990,14 @@ def eventFilter(self, obj, event):
         if t == QEvent.MouseButtonPress:
             if event.button() == Qt.LeftButton:
                 self._on_left_click()
-        # HERE BE DRAGONS. Usually I'd wrap this in an if not self.download,
-        # but for reasons not entirely clear, this caused a crash. The
-        # following odd way of expressing the same conditional doesn't cause a
-        # crash. Go figure... :-/
+        # HERE BE DRAGONS.
+        # The checks for "not self.downloading" in the following lines of code
+        # probably look strange. The obvious thing to do is to wrap such a
+        # check as a conditional. However, for reasons that are not entirely
+        # clear, this regularly caused random crashes when running the test
+        # suite (on Ubuntu 18.04, Python3.7). The following code expresses the
+        # same logic but without causing the random the crash of the test
+        # suite. 
         if (t == QEvent.HoverEnter or t == QEvent.HoverMove) and not self.downloading:
             self.download_button.setIcon(load_icon('download_file_hover.svg'))
         elif t == QEvent.HoverLeave and not self.downloading: