Headers can't expand back out when contracted all the way.

[redelman431]

Describe the bug
Once headers of gantt chart are contracted all the way, they can't be expanded back out again.

To Reproduce
Steps to reproduce the behavior:

1. Load example Gantt chart in examples/index.html

2. Drag one of the headers (Either "%", "Type", "Start", etc), contract them to the left until they are no longer visible.

3. Try to expand those contracted headers back out again and try to make them visible.

Expected behavior
The contracted headers should be made visible again by dragging them back out to the right. Instead they stay hidden until you refresh the page.

Desktop (please complete the following information):

• OS: Fedora Linux
• Browser: Firefox Quantum Version 60.0 64 bit.