diff --git a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-utils.ts b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-utils.ts
index e1de348..d309556 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-utils.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-utils.ts
@@ -8,6 +8,7 @@
 import {
   DiffPreferencesInfo,
   DiffResponsiveMode,
+  DisplayLine,
   FILE,
   LOST,
   LineNumber,
@@ -159,6 +160,87 @@
   return range;
 }
 
+/**
+ * This is all the data that gr-diff extracts from comment thread elements.
+ * Otherwise gr-diff treats such elements as a black box.
+ */
+export interface GrDiffCommentThread {
+  side: Side;
+  line: LineNumber;
+  range?: CommentRange;
+  rootId?: string;
+}
+
+export function toCommentThreadModel(
+  threadEl: HTMLElement
+): GrDiffCommentThread | undefined {
+  if (!isThreadEl(threadEl)) return undefined;
+  const side = getSide(threadEl);
+  const line = getLine(threadEl);
+  const range = getRange(threadEl);
+  if (!side) return undefined;
+  if (!line) return undefined;
+  return {side, line, range, rootId: threadEl.rootId};
+}
+
+export interface KeyLocations {
+  left: {[key: string]: boolean};
+  right: {[key: string]: boolean};
+}
+
+export function computeKeyLocations(
+  lineOfInterest: DisplayLine | undefined,
+  comments: GrDiffCommentThread[]
+) {
+  const keyLocations: KeyLocations = {left: {}, right: {}};
+
+  if (lineOfInterest) {
+    keyLocations[lineOfInterest.side][lineOfInterest.lineNum] = true;
+  }
+
+  for (const comment of comments) {
+    keyLocations[comment.side][comment.line] = true;
+    if (comment.range?.start_line) {
+      keyLocations[comment.side][comment.range.start_line] = true;
+    }
+  }
+
+  return keyLocations;
+}
+
+export function compareComments(
+  c1: GrDiffCommentThread,
+  c2: GrDiffCommentThread
+): number {
+  if (c1.side !== c2.side) {
+    return c1.side === Side.RIGHT ? 1 : -1;
+  }
+
+  if (c1.line !== c2.line) {
+    if (c1.line === FILE && c2.line !== FILE) return -1;
+    if (c1.line !== FILE && c2.line === FILE) return 1;
+    if (c1.line === LOST && c2.line !== LOST) return -1;
+    if (c1.line !== LOST && c2.line === LOST) return 1;
+    return (c1.line as number) - (c2.line as number);
+  }
+
+  if (c1.rootId !== c2.rootId) {
+    if (!c1.rootId) return -1;
+    if (!c2.rootId) return 1;
+    return c1.rootId > c2.rootId ? 1 : -1;
+  }
+
+  if (c1.range && c2.range) {
+    const r1 = JSON.stringify(c1.range);
+    const r2 = JSON.stringify(c2.range);
+    return r1 > r2 ? 1 : -1;
+  }
+  if (c1.range) return 1;
+  if (c2.range) return -1;
+
+  return 0;
+}
+
 // TODO: This type should be exposed to gr-diff clients in a separate type file.
 // For Gerrit these are instances of GrCommentThread, but other gr-diff users
 // have different HTML elements in use for comment threads.