PR Summary

This PR fixes two bugs with blitting Selectors:

1. A recursive paint loop when resizing the figure with the qt backend.
2. Fix a similar issue as describe in Update background in SpanSelector._press whenever useblit=True. #9660 but for RectangleSelector/EllipseSelector and SpanSelector once Interactive span selector improvement #20113 is merged, therefore fixing Update background in SpanSelector._press whenever useblit=True. #9660 too!

To reproduce 1

Fix in 94a2f46
From a juptyer qtconsole:

%matplotlib qt
import matplotlib.pyplot as plt
from matplotlib.widgets import RectangleSelector
import numpy as np

values = np.arange(100)

fig = plt.figure()
ax = fig.add_subplot()
ax.plot(values)

span = RectangleSelector(ax, print, useblit=True, interactive=True)

# flush events to get a first draw of the figure before setting the extents
fig.canvas.flush_events()
span.extents = (26, 55, 32, 71)

When resizing the figure, the figure is flickering (in spyder the console crashes when a custom DPI scaling is used) and it gives the following error:

QWidget::repaint: Recursive repaint detected
QWidget::paintEngine: Should no longer be called
QPainter::begin: Paint device returned engine == 0, type: 1
QPainter::end: Painter not active, aborted

My understanding is that calling _SelectorWidget.ax.blit in _SelectorWidget.update_background (through _SelectorWidget.update) when resizing the figure create the recursive repaint loop.

To reproduce 2

Fix in 031f77c
Adapted from #9660 to reproduce the issue with RectangleSelector and this PR uses the fix suggested in #9660 (comment)

import sys
from PyQt5.QtWidgets import *
from matplotlib.backends.backend_qt5agg import *
from matplotlib.figure import Figure
from matplotlib.widgets import RectangleSelector

class App(QMainWindow):
    def __init__(self):
        super().__init__()
        self.setCentralWidget(QWidget())
        layout = QVBoxLayout()
        self.centralWidget().setLayout(layout)
        self._fig = Figure()
        self._fig.subplots()
        layout.addWidget(FigureCanvas(self._fig))
        layout.addWidget(QPushButton("span", clicked=self._span_cb))

    def _span_cb(self):
        self._span = RectangleSelector(
            self._fig.axes[0], print, useblit=True)

qapp = QApplication(sys.argv)
app = App()
app.show()
qapp.exec_()

The selector is included in the background:

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).