When the chevron opens the details, the To/Cc rows pop up as an
absolutely-positioned panel anchored to the bottom of the meta
block — with bg, border, rounded corners and a shadow. Nothing
in the rest of the header reflows: From row stays put, the action
cluster stays put, the email body content stays put. This kills
all the height-/spacing-jump quirks the inline-expanded design
was fighting.
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