Backport PR #17877 on branch v3.3.x (Fix drawing zoom rubberband on GTK backends.)

doc/users/prev_whats_new/whats_new_3.3.0.rst

@@ -612,8 +612,8 @@ Previously, the x/y position displayed by the cursor text would usually include
 far more significant digits than the mouse pointing precision (typically one
 pixel). This is now fixed for linear scales.
 
-Qt zoom rectangle now black and white
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+GTK / Qt zoom rectangle now black and white
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 This makes it visible even over a dark background.
 

lib/matplotlib/backends/backend_gtk3.py

@@ -159,12 +159,14 @@ def __init__(self, figure):
 
         self._idle_draw_id = 0
         self._lastCursor = None
+        self._rubberband_rect = None
 
         self.connect('scroll_event',         self.scroll_event)
         self.connect('button_press_event',   self.button_press_event)
         self.connect('button_release_event', self.button_release_event)
         self.connect('configure_event',      self.configure_event)
         self.connect('draw',                 self.on_draw_event)
+        self.connect('draw',                 self._post_draw)
         self.connect('key_press_event',      self.key_press_event)
         self.connect('key_release_event',    self.key_release_event)
         self.connect('motion_notify_event',  self.motion_notify_event)
@@ -286,6 +288,40 @@ def configure_event(self, widget, event):
         self.figure.set_size_inches(w / dpi, h / dpi, forward=False)
         return False  # finish event propagation?
 
+    def _draw_rubberband(self, rect):
+        self._rubberband_rect = rect
+        # TODO: Only update the rubberband area.
+        self.queue_draw()
+
+    def _post_draw(self, widget, ctx):
+        if self._rubberband_rect is None:
+            return
+
+        x0, y0, w, h = self._rubberband_rect
+        x1 = x0 + w
+        y1 = y0 + h
+
+        # Draw the lines from x0, y0 towards x1, y1 so that the
+        # dashes don't "jump" when moving the zoom box.
+        ctx.move_to(x0, y0)
+        ctx.line_to(x0, y1)
+        ctx.move_to(x0, y0)
+        ctx.line_to(x1, y0)
+        ctx.move_to(x0, y1)
+        ctx.line_to(x1, y1)
+        ctx.move_to(x1, y0)
+        ctx.line_to(x1, y1)
+
+        ctx.set_antialias(1)
+        ctx.set_line_width(1)
+        ctx.set_dash((3, 3), 0)
+        ctx.set_source_rgb(0, 0, 0)
+        ctx.stroke_preserve()
+
+        ctx.set_dash((3, 3), 3)
+        ctx.set_source_rgb(1, 1, 1)
+        ctx.stroke()
+
     def on_draw_event(self, widget, ctx):
         # to be overwritten by GTK3Agg or GTK3Cairo
         pass
@@ -507,26 +543,14 @@ def set_cursor(self, cursor):
         Gtk.main_iteration()
 
     def draw_rubberband(self, event, x0, y0, x1, y1):
-        # adapted from
-        # http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/189744
-        ctx = self.canvas.get_property("window").cairo_create()
-
-        # todo: instead of redrawing the entire figure, copy the part of
-        # the figure that was covered by the previous rubberband rectangle
-        self.canvas.draw()
-
         height = self.canvas.figure.bbox.height
         y1 = height - y1
         y0 = height - y0
-        w = abs(x1 - x0)
-        h = abs(y1 - y0)
-        rect = [int(val) for val in (min(x0, x1), min(y0, y1), w, h)]
+        rect = [int(val) for val in (x0, y0, x1 - x0, y1 - y0)]
+        self.canvas._draw_rubberband(rect)
 
-        ctx.new_path()
-        ctx.set_line_width(0.5)
-        ctx.rectangle(rect[0], rect[1], rect[2], rect[3])
-        ctx.set_source_rgb(0, 0, 0)
-        ctx.stroke()
+    def remove_rubberband(self):
+        self.canvas._draw_rubberband(None)
 
     def _update_buttons_checked(self):
         for name, active in [("Pan", "PAN"), ("Zoom", "ZOOM")]:
@@ -721,6 +745,10 @@ def draw_rubberband(self, x0, y0, x1, y1):
         NavigationToolbar2GTK3.draw_rubberband(
             self._make_classic_style_pseudo_toolbar(), None, x0, y0, x1, y1)
 
+    def remove_rubberband(self):
+        NavigationToolbar2GTK3.remove_rubberband(
+            self._make_classic_style_pseudo_toolbar())
+
 
 class SaveFigureGTK3(backend_tools.SaveFigureBase):
     def trigger(self, *args, **kwargs):