Avoid "dash motion" in qt zoom box.

anntzer · anntzer · commit 35c1bfd1ff9a · 2020-05-01T23:05:24.000+02:00
Drawing the whole zoom box as a single rectangle causes the dashes on
the "last" edge of the rectangle to keep shifting (basically depending
on the total length of the other three edges).  Instead, draw the zoom
box as 4 separate lines always going from the "fixed" point (x0, y0)
towards to "mobile" point (x1, y1) so to avoid this effect.
diff --git a/lib/matplotlib/backends/backend_qt5.py b/lib/matplotlib/backends/backend_qt5.py
@@ -478,16 +478,23 @@ def drawRectangle(self, rect):
         # Draw the zoom rectangle to the QPainter.  _draw_rect_callback needs
         # to be called at the end of paintEvent.
         if rect is not None:
+            x0, y0, w, h = [pt / self._dpi_ratio for pt in rect]
+            x1 = x0 + w
+            y1 = y0 + h
             def _draw_rect_callback(painter):
-                scaled_rect = [pt / self._dpi_ratio for pt in rect]
                 pen = QtGui.QPen(QtCore.Qt.black, 1 / self._dpi_ratio)
                 pen.setDashPattern([3, 3])
-                painter.setPen(pen)
-                painter.drawRect(*scaled_rect)
-                pen.setDashOffset(3)
-                pen.setColor(QtCore.Qt.white)
-                painter.setPen(pen)
-                painter.drawRect(*scaled_rect)
+                for color, offset in [
+                        (QtCore.Qt.black, 0), (QtCore.Qt.white, 3)]:
+                    pen.setDashOffset(offset)
+                    pen.setColor(color)
+                    painter.setPen(pen)
+                    # Draw the lines from x0, y0 towards x1, y1 so that the
+                    # dashes don't "jump" when moving the zoom box.
+                    painter.drawLine(x0, y0, x0, y1)
+                    painter.drawLine(x0, y0, x1, y0)
+                    painter.drawLine(x0, y1, x1, y1)
+                    painter.drawLine(x1, y0, x1, y1)
         else:
             def _draw_rect_callback(painter):
                 return
