Avoid triggering full dpi change if it's unchanged.

QuLogic · QuLogic · commit 8feb6d8fafe1 · 2020-03-30T20:05:11.000-04:00
This is essentially a revert of the `Text.get_window_extent` change to its dpi handling, but applies in _all_ cases. Depending on backend, we want to avoid this extraneous dpi change, because it can cause recursive event signals. For example, with GTK3, a resize will trigger its event loop, which eventually causes a re-draw. The re-draw would get to `Text.get_window_extent`, change the dpi (to the same value), and call `Figure.set_size_inches`, which would resize the window again. Depending on how much of the GTK event loop was processed, this may or may not trigger another draw, recursing again and again. Fixes #16934.
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -442,6 +442,9 @@ def _set_dpi(self, dpi, forward=True):
         forward : bool
             Passed on to `~.Figure.set_size_inches`
         """
+        if dpi == self._dpi:
+            # We don't want to cause undue events in backends.
+            return
         self._dpi = dpi
         self.dpi_scale_trans.clear().scale(dpi)
         w, h = self.get_size_inches()
