Don't clip colorbar dividers

[QuLogic]

PR Summary

They are sometimes right on the edge of the Axes, and the last (or possibly first) don't get drawn due to clipping. Because the divider line width is the same as the Axes frame line width, we don't have to worry about it going outside when unclipped.

Fixes #22864

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• [n/a] New features are documented, with examples if plot related.
• [n/a] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [n/a] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• [n/a] Documentation is sphinx and numpydoc compliant (the docs should build without error).

[jklymak]

I think we can zoom in on colorbar axes now, though, and we can certainly set the x/ylimits. Won't these now dangle in space if we do that?

[QuLogic]

I was thinking that, but when I tried it with the original example, I was unable to pan or zoom the colorbar. Did it break, or is the test not sufficiently set up to allow it?

[greglucas]

I don't see these dangling in space if I test the original example with this...

import matplotlib as mpl
import numpy as np
import matplotlib.pyplot as plt  
from matplotlib.colors import from_levels_and_colors

my_cmap = mpl.cm.viridis
bounds = np.arange(10)
nb_colors = len(bounds) + 1
colors = my_cmap(np.linspace(100, 255, nb_colors).astype(int))
my_cmap, my_norm = from_levels_and_colors(bounds, colors, extend='both')

plt.figure(figsize=(5, 1))
ax = plt.subplot(111)
cbar = mpl.colorbar.ColorbarBase(ax, cmap=my_cmap, norm=my_norm, orientation='horizontal', drawedges=True)
cbar.ax.set_xlim(2, 10)
cbar.ax.set_navigate(True)
plt.subplots_adjust(left=0.05, bottom=0.4, right=0.95, top=0.9)
plt.show()

Because it is a BoundaryNorm we disable interaction:

matplotlib/lib/matplotlib/colorbar.py

Lines 474 to 477 in 02e93e2

	# Don't navigate on any of these types of mappables
	if (isinstance(self.norm, (colors.BoundaryNorm, colors.NoNorm)) or
	isinstance(self.mappable, contour.ContourSet)):
	self.ax.set_navigate(False)

Manually enabling interaction moves the colors, but not the boundary lines, so those will not move around either apparently?

[QuLogic]

Going back to what pan/zoom on the Colorbar means, it moves the norm limits, and then moves the axis to match, so the colours and the dividers never move.

Calling set_xlim can break things though:

[QuLogic]

This now tests with set_xlim as well as the other vertical orientation. It should produce a divider at the right places now even when the limits have been changed.