A step that removes a node has nothing left to highlight in the resulting text,
so range resolution fell back to the enclosing block and flooded it. Record the
changed node's surviving neighbours as seam anchors (captured before the
mutation) and split a step's candidates into precise / seam / ancestor tiers:
when neither the node nor its marker resolves, place a zero-length caret at the
gap -- the successor's start, else the predecessor's end -- and only fall back
to the enclosing block when no neighbour survives. A zero-length highlight is
rendered as a caret (positioned, pulsed, centered) with no background mark.
Assisted-by: Claude:claude-opus-4-8:Claude Code
Siegfried Pammer