fix for container calculations

[Traksewt]

Calculating the end location was checking all parents. It should stop when it reaches the given container.

The window.internalHeight was interfering with the container animation exit calculations.

A threshold is added to stop the animation going on for a long time when the difference is less than a pixel.

[Traksewt]

adding package.json for webpack installation.

[samsullivan]

Thanks! This was much better than the fix I came up with (which didn't take into account any potential offsets).