Similar feature exists in Lister for a long time, but was omitted in comparison.
Possible complications could be:
1. both panes may have selections;
2. multi-line selections.
Simple solutions.
1. Choose active pane in case of selections on both sides.
2. It would still be good, if the feature was implemented only in Edit mode.
Othewise, use first non-empty line in multi-line selection.
Compare by content: use selection as search text
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