add_lines misses lines for matplotlib.colorbar.ColorbarBase #12458

Bug report

Bug summary

For matplotlib.colorbar.ColorbarBase(...) usage of add_lines(...) may yield missing lines/poorly adjusted lines.

Code for reproduction

import matplotlib.pyplot as plt
import matplotlib as mpl
import numpy as np

fig = plt.figure()
ax = fig.add_axes([0.05, 0.8, 0.9, 0.15])
cmap = mpl.cm.cool
norm = mpl.colors.Normalize(vmin=-4, vmax=4)
levels = (-1.0, 1.0, 2.0, 3.0)

cb = mpl.colorbar.ColorbarBase(ax, cmap=cmap, norm=norm)
colors_bg = np.tile(list((1.0, 1.0, 1.0, 1.0)), (len(levels), 1))
cb.add_lines(levels=levels, colors=colors_bg, linewidths=7)

plt.show()

Expected outcome

4 white lines, crossing the colorbar.
Used to work on matplotlib 2.2.3!

Matplotlib version

• Operating system: Ubuntu 18.04 LTS
• Matplotlib version: 3.0.0
• Matplotlib backend (print(matplotlib.get_backend())): Qt5Agg
• Python version: 3.7.0

Miniconda install, fully updated.