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web/src/lib/anchor.ts

@@ -106,16 +106,24 @@ export function isOutdated(c: Comment, anchors: DiffAnchors | null): boolean {
 // changeKeyIndex maps "side:line" -> react-diff-view change key for one file's
 // hunks, so threads and composers can be attached as line widgets. Built from
 // the hunks actually being rendered (expansion included), unlike buildAnchors.
+//
+// `newOnly` is for the since-viewed diff, whose old side is a snapshot of the
+// file held in this browser rather than the base ref. Those line numbers are
+// real, but they number a revision no comment was ever written against, so they
+// must not be offered as anchors — indexing them would hang a thread meant for
+// base line 40 off whatever line 40 of the snapshot happens to be.
 export function changeKeyIndex(
   hunks: readonly { changes: ChangeData[] }[],
+  newOnly = false,
 ): Record<string, string> {
   const map: Record<string, string> = {};
   for (const hunk of hunks) {
     for (const change of hunk.changes) {
       const key = getChangeKey(change);
       const nl = lineFor(change, 'new');
-      const ol = lineFor(change, 'old');
       if (nl != null) map[`new:${nl}`] = key;
+      if (newOnly) continue;
+      const ol = lineFor(change, 'old');
       if (ol != null) map[`old:${ol}`] = key;
     }
   }