Zoom broken in some demos

[siddhantwahal]

Problem Description
This bug affects the following demos:

• shell/contour.py
• shell/contourf.py
• basic/contour_plot.py

Mouse-scroll doesn't zoom the plot until the user pans the plot. After panning, the plot is really zoomed out.

Reproduction Steps:
Mouse-scroll, then left-drag.

python shell/contour.py

Expected behavior:
Plot should zoom immediately on mouse-scroll.

OS, Python version:
MacOSX 10.14
Python3.6