fix(diagram/header): fix how expanded height is computed

Merged Dave Patrick Caberto requested to merge SeaDve/bustle:expanded-height into rustle

This fixes cases where line's height extent can be greater than the entire layout's height extent, causing difference in default and expanded height despite a maximum line count of 1 across layouts.

Requires !56 (merged) and !57 (merged)

Edited by Dave Patrick Caberto

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