Fix RangeSlider.set_val when outside existing value

The clipping in the setter ensures that min <= max, but if you are
setting both values outside the existing range, this will incorrectly
set new minimum to old maximum and vice versa.

The tests also passed accidentally because all the new ranges overlapped
with the old, and did not trigger this issue.

Fixes matplotlib#25338

Author: QuLogic

lib/matplotlib/tests/test_widgets.py

@@ -1297,12 +1297,12 @@ def handle_positions(slider):
         else:
             return [h.get_xdata()[0] for h in slider._handles]
 
-    slider.set_val((0.2, 0.6))
-    assert_allclose(slider.val, (0.2, 0.6))
-    assert_allclose(handle_positions(slider), (0.2, 0.6))
+    slider.set_val((0.4, 0.6))
+    assert_allclose(slider.val, (0.4, 0.6))
+    assert_allclose(handle_positions(slider), (0.4, 0.6))
 
     box = slider.poly.get_extents().transformed(ax.transAxes.inverted())
-    assert_allclose(box.get_points().flatten()[idx], [0.2, .25, 0.6, .75])
+    assert_allclose(box.get_points().flatten()[idx], [0.4, .25, 0.6, .75])
 
     slider.set_val((0.2, 0.1))
     assert_allclose(slider.val, (0.1, 0.2))

lib/matplotlib/widgets.py

@@ -702,7 +702,7 @@ def __init__(
                          valmin, valmax, valfmt, dragging, valstep)
 
         # Set a value to allow _value_in_bounds() to work.
-        self.val = [valmin, valmax]
+        self.val = (valmin, valmax)
         if valinit is None:
             # Place at the 25th and 75th percentiles
             extent = valmax - valmin
@@ -947,9 +947,9 @@ def set_val(self, val):
         """
         val = np.sort(val)
         _api.check_shape((2,), val=val)
-        vmin, vmax = val
-        vmin = self._min_in_bounds(vmin)
-        vmax = self._max_in_bounds(vmax)
+        # Reset value to allow _value_in_bounds() to work.
+        self.val = (self.valmin, self.valmax)
+        vmin, vmax = self._value_in_bounds(val)
         self._update_selection_poly(vmin, vmax)
         if self.orientation == "vertical":
             self._handles[0].set_ydata([vmin])