From cae28cba9cc43e2fc6a60d4c579042b521f4b717 Mon Sep 17 00:00:00 2001
From: Antony Lee <anntzer.lee@gmail.com>
Date: Wed, 1 Nov 2017 17:50:07 -0700
Subject: [PATCH] Update background in SpanSelector._press whenever
 useblit=True.

Previously, the canvas would only get redrawn when useblit=True only if
span_stays is set.  This causes e.g.

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

    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 = SpanSelector(
                self._fig.axes[0], print, "horizontal", useblit=True)

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

to use an incorrect background(?) when clicking and starting a span for
the first time.

To be honest I'm not sure this is exactly the correct solution, although
it does fix the issue...
---
 lib/matplotlib/widgets.py | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/lib/matplotlib/widgets.py b/lib/matplotlib/widgets.py
index 5ac6986926f6..7ab310a8c941 100644
--- a/lib/matplotlib/widgets.py
+++ b/lib/matplotlib/widgets.py
@@ -1811,10 +1811,8 @@ def _press(self, event):
         self.rect.set_visible(self.visible)
         if self.span_stays:
             self.stay_rect.set_visible(False)
-            # really force a draw so that the stay rect is not in
-            # the blit background
-            if self.useblit:
-                self.canvas.draw()
+        if self.useblit:
+            self.canvas.draw()
         xdata, ydata = self._get_data(event)
         if self.direction == 'horizontal':
             self.pressv = xdata