Zoom looses its focus after all graphs on page rendered

[cigamit]

Describe the bug
If you are zooming too quickly, before a page is finally rendered, the zoom window will be erased in the middle of your zoom operation.

To Reproduce
Steps to reproduce the behavior:

1. Render a page with several dozen graphs
2. While graphs are still rendering, start a zoom on one of the existing graphs.
3. Wait to see the zoom window disappear

Expected behavior
Zooming should be allowed even though not all graphs on a page have been rendered already.