Bug report

Bug summary
Playback of mp4 files written by FFMpegWriter shows a rolling image if the dpi setting does not match that specified when a figure is created. In particular, this happens if the movie frame images are being displayed on the screen, e.g. during creation of the movie. (There is also an annoying flicker.)

Code for reproduction

Animation in the code below fails, resulting in a rolling image on playback of the saved mp4 movie. Either of two modifications indicated in the comments make the saved movie play back normally.

import matplotlib.animation
import numpy
import pylab

fig = pylab.figure(figsize=(6,6), dpi=120)
writer = matplotlib.animation.FFMpegWriter(fps=10)
x = numpy.linspace(0, 6.28, 100)

with writer.saving(fig, 'rolling_bug.mp4', dpi=100):           # saving OK if dpi=120
    for offset in numpy.linspace(0, 6.28, 30):
        pylab.plot(numpy.cos(x-offset), color='black')
        # saving OK if the next line is commented out
        pylab.pause(0.01)
        writer.grab_frame()

Note that commenting out the pylab.pause() call results in no image being displayed on the screen until the end of the script.

Actual outcome
Inspection of the figure vertical dimension in pixels shows that it has changed across the call to grab_frame(), from 600 to 590 in the above example. The horizontal dimension does not change.
A screen grab from VLC Media Player is shown here:

Expected outcome

The writer.grab_frame() should not change the vertical dimension of the figure.

The flickering did not occur in the last version of Matplotlib I had installed, but I do not know what version that was. I'm pretty sure the vertical image dimension also stayed constant across the grab_frame() call, since this was (abstracted from) working code from about a year ago.

Version info

Windows 8.1
Matplotlib 2.0.2
Python 3.6.1
Anaconda 4.4.0

Installed from conda