The composer's paste path used RangeSelection.insertText, which keeps every
pasted newline inside one text node. In that document the browser reports the
collapsed caret's geometry as the whole block's, so deleting the draft's last
line moved the capped scrollport to the draft's head while the caret stayed at
its end.
RangeSelection.insertRawText splits the input into text and line-break nodes,
the document a typed draft already has.
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