Fixed layer (amp-sidebar) - reset state when elements are moved out of fixed layer

[camelburrito]

Short description of your issue:

When you move elements back and forth between the fixed layer then the computations are stacked up and it causes weirdness like this

https://www.youtube.com/watch?v=IsDShWCA7Ho&feature=youtu.be

What browsers are affected?

IOS (safari?) - where ever fixed layer is being used.

[camelburrito]

cc @dvoytenko

[ericlindley-g]

@sriramkrish85 Tentatively slotting for 4/14, but is that too aggressive for this fix?

[camelburrito]

@ericlindley-g hopefully! - working on it now.