FIX: always eraseRect in Qt widget (matplotlib#13050)

Re-focusing the figure window will call the draw method.  If the
facecolor of the figure and axes are transparent and we do not
erase the widget we are effectively compositing the figure on to its
self which results in artifacts anywhere there is alpha or
anti-aliasing.

Closes matplotlib#13012

Author: tacaswell

lib/matplotlib/backends/backend_qt5.py

@@ -240,7 +240,6 @@ def __init__(self, figure):
         self._dpi_ratio_prev = None
 
         self._draw_pending = False
-        self._erase_before_paint = False
         self._is_drawing = False
         self._draw_rect_callback = lambda painter: None
 
@@ -491,7 +490,6 @@ def draw(self):
             return
         with cbook._setattr_cm(self, _is_drawing=True):
             super().draw()
-        self._erase_before_paint = True
         self.update()
 
     def draw_idle(self):

lib/matplotlib/backends/backend_qt5agg.py

@@ -38,10 +38,6 @@ def paintEvent(self, event):
 
         painter = QtGui.QPainter(self)
 
-        if self._erase_before_paint:
-            painter.eraseRect(self.rect())
-            self._erase_before_paint = False
-
         rect = event.rect()
         left = rect.left()
         top = rect.top()
@@ -55,6 +51,10 @@ def paintEvent(self, event):
         reg = self.copy_from_bbox(bbox)
         buf = cbook._unmultiplied_rgba8888_to_premultiplied_argb32(
             memoryview(reg))
+
+        # clear the widget canvas
+        painter.eraseRect(rect)
+
         qimage = QtGui.QImage(buf, buf.shape[1], buf.shape[0],
                               QtGui.QImage.Format_ARGB32_Premultiplied)
         if hasattr(qimage, 'setDevicePixelRatio'):

lib/matplotlib/backends/backend_qt5cairo.py

@@ -37,9 +37,7 @@ def paintEvent(self, event):
             # Not available on Qt4 or some older Qt5.
             qimage.setDevicePixelRatio(dpi_ratio)
         painter = QtGui.QPainter(self)
-        if self._erase_before_paint:
-            painter.eraseRect(self.rect())
-            self._erase_before_paint = False
+        painter.eraseRect(event.rect())
         painter.drawImage(0, 0, qimage)
         self._draw_rect_callback(painter)
         painter.end()