Bug fix/occluding viewer panes

[nstrayer]

Intent

Fix problem where viewer and help-pane can occlude other parts of the editor.

Addresses #1997

Approach

This issue was caused by there being a minimum height for the vertical size of a ViewPane of 120 pixels.

This was causing the layout method to never tell the editor to be smaller than 120px tall.

[jmcphers]

What is the observed size when this happens? I wonder if another way to do this would be to disown/detach the overlay webview when the size is sufficiently small.

Totally fine with this workaround for now, though.

[nstrayer]

There are cases where it would look weird if the webview disappeared at smaller than 120px sizes. I'll add a note though about potentially detaching when the size gets to the point where nothing is visible, although maybe the cost of disposing and then reopening could be greater than just leaving a short webview around? I'm not familiar with the overhead, but we wont ever have an unbounded number sitting there.