Add note on incompatibility with blitting of widgets

Author: timhoffm

doc/release/next_whats_new/pyplot-register-figure.rst

@@ -58,3 +58,6 @@ general, you should not store a reference to the canvas, but rather always
 obtain it from the figure with ``fig.canvas``. This will return the current
 canvas, which is either the original `.FigureCanvasBase` or a backend-dependent
 subclass, depending on whether the figure is registered with pyplot or not.
+Additionally, the swapping of the canvas currently does not play well with
+blitting of matplotlib widgets; in such cases either deactivate blitting or do not
+continue to use the figure (e.g. saving it after closing the window).

lib/matplotlib/tests/test_backends_interactive.py

@@ -18,6 +18,7 @@
 
 import matplotlib as mpl
 from matplotlib import _c_internal_utils
+from matplotlib.backend_bases import FigureCanvasBase
 from matplotlib.backend_tools import ToolToggleBase
 from matplotlib.testing import subprocess_run_helper as _run_helper, is_ci_environment
 
@@ -229,6 +230,7 @@ def check_alt_backend(alt_backend):
 
     # When the figure is closed, it's manager is removed and the canvas is reset to
     # FigureCanvasBase. Saving should still be possible.
+    assert type(fig.canvas) == FigureCanvasBase
     result_after = io.BytesIO()
     fig.savefig(result_after, format='png', dpi=100)