Add check for syntax highlighting in diff prefs changed

Release-Notes: skip
Google-bug-id: b/233199790
Change-Id: I1883813f11801e5bbddc033fc149731c102dc925
diff --git a/polygerrit-ui/app/elements/shared/gr-diff-preferences/gr-diff-preferences.ts b/polygerrit-ui/app/elements/shared/gr-diff-preferences/gr-diff-preferences.ts
index cb51337..7d52a3c 100644
--- a/polygerrit-ui/app/elements/shared/gr-diff-preferences/gr-diff-preferences.ts
+++ b/polygerrit-ui/app/elements/shared/gr-diff-preferences/gr-diff-preferences.ts
@@ -305,6 +305,8 @@
     // We have to wrap boolean values in Boolean() to ensure undefined values
     // use false rather than undefined.
     return (
+      Boolean(this.originalDiffPrefs?.syntax_highlighting) !==
+        Boolean(this.diffPrefs?.syntax_highlighting) ||
       this.originalDiffPrefs?.context !== this.diffPrefs?.context ||
       Boolean(this.originalDiffPrefs?.line_wrapping) !==
         Boolean(this.diffPrefs?.line_wrapping) ||