The snapshot viewport size would change by:

• The user or script resizing the window.
• A mobile device changing orientation.

This impacts a transition if it happens:

• Between capturing the old and new states.
• During the transition animation.

Options:

Skip the transition

A resize creates situations the developer may not have predicted and catered for. For example, they may have assumed that their site header would be the same size in the old and new states. A resize can break that assumption.

Given that the transition is an enhancement, and the user is performing an 'interrupting' action by resizing the window, skipping the transition seems reasonable.

Update the animation state and continue

All 'new' elements will be updated with their new size, which in turn will update all animations in progress. This includes the transition container representing the root element - this will be the only case where this element animates width & height.