The goal is to make the editor behave like a standard block-level
element.
The horizontal behavior is simple: we stretch horizontally to fill our
container.
The vertical behavior is more nuanced. If an explicit height is assigned
on the wrapper view, we honor that height. But if no explicit height is
assigned, the editor stretches vertically so that its contents are
visible.
This prepares us to support mini editors, which need to be 1-line tall
without an explicit height assignment.
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