A pasted link linkifies into an `@url:` directive, so pasting one while an
@url: scope was already open stacked a second directive on the first and
submitted `@url:@url:` around the link. The leftover prefix rendered as
literal text in front of the chip.
insertComposerContentsAtCaret takes a consumeBefore length; both composers
pass the open scope's span so the scope is consumed by the paste rather than
left sitting in front of the chip.
Brooklyn Nicholson
130cc0b859