diff --git a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
index 09e9d7c..b42deda 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
@@ -23,10 +23,12 @@
   GeneratedWebLink,
 } from '../../core/gr-navigation/gr-navigation';
 import {
+  anyLineTooLong,
   getLine,
   getRange,
   getSide,
   rangesEqual,
+  SYNTAX_MAX_LINE_LENGTH,
 } from '../gr-diff/gr-diff-utils';
 import {appContext} from '../../../services/app-context';
 import {
@@ -95,10 +97,6 @@
 // Disable syntax highlighting if the overall diff is too large.
 const SYNTAX_MAX_DIFF_LENGTH = 20000;
 
-// If any line of the diff is more than the character limit, then disable
-// syntax highlighting for the entire file.
-const SYNTAX_MAX_LINE_LENGTH = 500;
-
 // 120 lines is good enough threshold for full-sized window viewport
 const NUM_OF_LINES_THRESHOLD_FOR_VIEWPORT = 120;
 
@@ -1030,7 +1028,7 @@
     if (!preferenceChangeRecord?.base?.syntax_highlighting || !diff) {
       return false;
     }
-    if (this._anyLineTooLong(diff)) {
+    if (anyLineTooLong(diff)) {
       fireAlert(
         this,
         `Files with line longer than ${SYNTAX_MAX_LINE_LENGTH} characters` +
@@ -1049,20 +1047,6 @@
     return true;
   }
 
-  /**
-   * @return whether any of the lines in diff are longer
-   * than SYNTAX_MAX_LINE_LENGTH.
-   */
-  _anyLineTooLong(diff?: DiffInfo) {
-    if (!diff) return false;
-    return diff.content.some(section => {
-      const lines = section.ab
-        ? section.ab
-        : (section.a || []).concat(section.b || []);
-      return lines.some(line => line.length >= SYNTAX_MAX_LINE_LENGTH);
-    });
-  }
-
   _listenToViewportRender() {
     const renderUpdateListener: DiffLayerListener = start => {
       if (start > NUM_OF_LINES_THRESHOLD_FOR_VIEWPORT) {
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-utils.ts b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-utils.ts
index 96fbd8d..fada9cb 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-utils.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff-utils.ts
@@ -18,6 +18,11 @@
 import {CommentRange} from '../../../types/common';
 import {FILE, LineNumber} from './gr-diff-line';
 import {Side} from '../../../constants/constants';
+import {DiffInfo} from '../../../types/diff';
+
+// If any line of the diff is more than the character limit, then disable
+// syntax highlighting for the entire file.
+export const SYNTAX_MAX_LINE_LENGTH = 500;
 
 /**
  * Compare two ranges. Either argument may be falsy, but will only return
@@ -130,3 +135,17 @@
     (node as Element).classList.contains('comment-thread')
   );
 }
+
+/**
+ * @return whether any of the lines in diff are longer
+ * than SYNTAX_MAX_LINE_LENGTH.
+ */
+export function anyLineTooLong(diff?: DiffInfo) {
+  if (!diff) return false;
+  return diff.content.some(section => {
+    const lines = section.ab
+      ? section.ab
+      : (section.a || []).concat(section.b || []);
+    return lines.some(line => line.length >= SYNTAX_MAX_LINE_LENGTH);
+  });
+}
diff --git a/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.ts b/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.ts
index 4e953e8..cf8a0ed 100644
--- a/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.ts
+++ b/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.ts
@@ -61,6 +61,7 @@
 import {GrSyntaxLayer} from '../../diff/gr-syntax-layer/gr-syntax-layer';
 import {StorageLocation} from '../../../services/storage/gr-storage';
 import {TokenHighlightLayer} from '../../diff/gr-diff-builder/token-highlight-layer';
+import {anyLineTooLong} from '../../diff/gr-diff/gr-diff-utils';
 
 const UNRESOLVED_EXPAND_COUNT = 5;
 const NEWLINE_PATTERN = /\n/g;
@@ -245,15 +246,17 @@
   get _diff() {
     if (this.comments === undefined || this.path === undefined) return;
     if (!this.comments[0]?.context_lines?.length) return;
-    waitForEventOnce(this, 'render').then(() => {
-      this.syntaxLayer.process();
-    });
     const diff = computeDiffFromContext(
       this.comments[0].context_lines,
       this.path,
       this.comments[0].source_content_type
     );
-    this.syntaxLayer.init(diff);
+    if (!anyLineTooLong(diff)) {
+      this.syntaxLayer.init(diff);
+      waitForEventOnce(this, 'render').then(() => {
+        this.syntaxLayer.process();
+      });
+    }
     return diff;
   }