This took awhile to understand and I thought I was going nuts trying to get OSX Core Graphics to clear a rectangle. It turns out the stale drawing was coming from Agg itself. After comparing against Qt5Agg, it turns out that before this PR, MacOSX never called get_renderer with cleared=True, which causes the Agg renderer to clear its buffer; it only happened on resize/dpi change. With this patch, we now pass the stale flag as the value for cleared.

Fixes #8282.

It also turns out that get_renderer was a complete copy of what's in FigureCanvasAgg, so I just nuked the method in FigureCanvasMac.

Testing was accomplished with a slight modification of the code from #8282:

import matplotlib.pyplot as plt
fig,ax = plt.subplots(facecolor='none')
ax.set_facecolor('none')

plt.subplots_adjust(left=0.25, bottom=0.25)
plt.axis([0, 4, 0,2])
l, = ax.plot([1,2,3],[1,0,1])
ax.set_title('down')
def update():
    l.set_ydata([1,2,1])
    ax.set_title('up')

plt.pause(1.0)
update()
plt.pause(1.0)
plt.show()

Before this patch, you get a diamond-shaped plot and titles overlaid. After, the whole plot changes between subsequent draws.