Very large entities are confusing to add - they appear for a brief moment
and then grow off the screen. This shows an arrow if an entity is at least
ten times taller than the viewport.

This also probably fixes bugs with wrong z-indexing for fresh entities

Having every single property affected by the css transition
meant that switching sources waited for the transition to
complete, and also required for the grabbed class (which
disabled transitions entirely while moving entities). Now,
only height and max-height are affected by the transitions.

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