FIX: when creating a canvas from a Figure use original dpi

When we upscale the DPI for high-dpi screens we stash the original dpi and then
set the figure dpi to the scaled version.  If the same Figure instance is
repeatedly passed to a Canvas that support hi-dpi it would go into a loop where
the scaled DPI is treated as the original dpi and infinitely increases.

By grabbing `fig._original_dpi` (which we stash for exactly this reason) we can
avoid this loop.

closes matplotlib#26380

Author: tacaswell

lib/matplotlib/backend_bases.py

@@ -1763,7 +1763,7 @@ def __init__(self, figure=None):
         self.toolbar = None  # NavigationToolbar2 will set me
         self._is_idle_drawing = False
         # We don't want to scale up the figure DPI more than once.
-        figure._original_dpi = figure.dpi
+        figure._original_dpi = getattr(figure, '_original_dpi', figure.dpi)
         self._device_pixel_ratio = 1
         super().__init__()  # Typically the GUI widget init (if any).
 

lib/matplotlib/figure.py

@@ -3003,12 +3003,10 @@ def _set_base_canvas(self):
         This is used upon initialization of the Figure, but also
         to reset the canvas when decoupling from pyplot.
         """
-        # check if we have changed the DPI due to hi-dpi screens
-        orig_dpi = getattr(self, '_original_dpi', self._dpi)
         FigureCanvasBase(self)  # Set self.canvas as a side-effect
-        # put it back to what it was
-        if orig_dpi != self._dpi:
-            self.dpi = orig_dpi
+        # undo any high-dpi scaling
+        if self._dpi != self._original_dpi:
+            self.dpi = self._original_dpi
 
     def set_canvas(self, canvas):
         """