Don't call draw() twice when Qt canvas first appears.

anntzer · anntzer · commit c67a94455892 · 2017-08-26T12:36:55.000-07:00
Right now the resizeEvent triggered from within the paintEvent will
itself trigger a second paintEvent.  Adding a print to draw() shows that
this patch ensures that draw() only gets called once the first time the
canvas appears.
diff --git a/lib/matplotlib/backends/backend_qt5agg.py b/lib/matplotlib/backends/backend_qt5agg.py
@@ -74,8 +74,7 @@ def paintEvent(self, e):
         # As described in __init__ above, we need to be careful in cases with
         # mixed resolution displays if dpi_ratio is changing between painting
         # events.
-        if (self._dpi_ratio_prev is None or
-            self._dpi_ratio != self._dpi_ratio_prev):
+        if self._dpi_ratio != self._dpi_ratio_prev:
             # We need to update the figure DPI
             self._update_figure_dpi()
             # The easiest way to resize the canvas is to emit a resizeEvent
@@ -87,6 +86,8 @@ def paintEvent(self, e):
             # straight away, and this causes visual delays in the changes.
             self.resizeEvent(event)
             self._dpi_ratio_prev = self._dpi_ratio
+            # resizeEvent triggers a paintEvent itself, so we exit this one.
+            return
 
         painter = QtGui.QPainter(self)
 
