`plainTextInRange` serialized the caret's preceding content through a bare
<div>, but `composerPlainText` appends "\n" to any block element that isn't the
editor slot. So `beforeText` always looked like it ended in whitespace and the
separating space was never inserted — dragging a file in after a word produced
`review@file:...` glued together.
Marking the scratch container with RICH_INPUT_SLOT makes it serialize in the
same coordinates as the editor. Same fix lands in the new `caretOffsetInEditor`,
which measures caret offsets the same way.
Brooklyn Nicholson
8896fc7500