Fix: De-activate animation when scroll container has no overflow

[dnlzro]

Problem

This polyfill applies scroll-driven animations even when the scroll container has no overflow (i.e., when clientHeight === scrollHeight). According to the specification (source):

Progress (the current time) for a scroll progress timeline is calculated as: scroll offset ÷ (scrollable overflow size − scroll container size)

If the 0% position and 100% position coincide (i.e. the denominator in the current time formula is zero), the timeline is inactive.

When there is no overflow, then scrollable overflow size − scroll container size = 0. Currently, the timeline is set to 100% or NaN in these cases. Instead, it should be set to inactive.

Changes

1. ScrollTimeline.currentTime: Return unresolved when maxScrollPos <= 0.
2. ViewTimeline.currentTime: Return unresolved when offsets.end === offsets.start.

Note: When the currentTime method returns unresolved (i.e., null), the tickAnimation function will cancel the animation. This ensures no effects are applied when the timeline is inactive.

Note: I think these changes also indirectly address the first issue raised in #298.

[boneyfantaseas]

Instantly no annoying "Animation.currentTime" errors anymore, lovely. I use it for a scrollbar indicator approach and I learned that it works this solution works with indicators as pseudo elements but not as real DOM nodes.