Fix a bug with the Qt5 backend with mixed resolution displays

astrofrog · astrofrog · commit f8cb5c9e47f2 · 2017-07-24T16:30:44.000+01:00
When mixed-resolution displays are present, the _dpi_ratio attribute on the canvas may change between paintEvents, which means that we need to change the size of the canvas. However, the underlying canvas is only resized if a resizeEvent is emitted, so we check whether _dpi_ratio has changed between paintEvents, and if so we emit a fake resizeEvent for the widget.
diff --git a/lib/matplotlib/backends/backend_qt5agg.py b/lib/matplotlib/backends/backend_qt5agg.py
@@ -38,6 +38,14 @@ def __init__(self, figure):
         self._bbox_queue = []
         self._drawRect = None
 
+        # In cases with mixed resolution displays, we need to be careful if the
+        # dpi_ratio changes - in this case we need to resize the canvas
+        # accordingly. We could watch for screenChanged events from Qt, but
+        # the issue is that we can't guarantee this will be emitted *before*
+        # the first paintEvent for the canvas, so instead we keep track of the
+        # dpi_ratio value here and in paintEvent we resize the canvas if needed.
+        self._dpi_ratio_prev = None
+
     def drawRectangle(self, rect):
         if rect is not None:
             self._drawRect = [pt / self._dpi_ratio for pt in rect]
@@ -56,6 +64,23 @@ def paintEvent(self, e):
         In Qt, all drawing should be done inside of here when a widget is
         shown onscreen.
         """
+
+        # 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:
+            self._dpi_ratio_prev = self._dpi_ratio
+        elif self._dpi_ratio != self._dpi_ratio_prev:
+            # The easiest way to resize the canvas is to emit a resizeEvent
+            # since we implement all the logic for resizing the canvas for
+            # that event.
+            event = QtGui.QResizeEvent(self.size(), self.size())
+            # We use self.resizeEvent here instead of QApplication.postEvent
+            # since the latter doesn't guarantee that the event will be emitted
+            # straight away, and this causes visual delays in the changes.
+            self.resizeEvent(event)
+            self._dpi_ratio_prev = self._dpi_ratio
+
         # if the canvas does not have a renderer, then give up and wait for
         # FigureCanvasAgg.draw(self) to be called
         if not hasattr(self, 'renderer'):
