diff --git a/tests/check_resize_observer.mjs b/tests/check_resize_observer.mjs
new file mode 100644
index 00000000..14eb70a7
--- /dev/null
+++ b/tests/check_resize_observer.mjs
@@ -0,0 +1,6 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+test("check if ResizeObserver exists on globalThis", () => {
+    console.log("globalThis.ResizeObserver:", typeof globalThis.ResizeObserver);
+});
diff --git a/tests/ui/components/debug_hashtag_strip_helper.test.mjs b/tests/ui/components/debug_hashtag_strip_helper.test.mjs
new file mode 100644
index 00000000..8546fa57
--- /dev/null
+++ b/tests/ui/components/debug_hashtag_strip_helper.test.mjs
@@ -0,0 +1,43 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { JSDOM } from "jsdom";
+import { HashtagStripHelper } from "../../../js/ui/components/hashtagStripHelper.js";
+
+function createDom(options = {}) {
+  const dom = new JSDOM("<!DOCTYPE html><body><div id='container'></div></body>", {
+    pretendToBeVisual: true,
+  });
+
+  const win = dom.window;
+  const doc = win.document;
+
+  if (options.useResizeObserver) {
+     win.ResizeObserver = class MockResizeObserver {
+         observe() {}
+         disconnect() {}
+     };
+  } else {
+     delete win.ResizeObserver;
+  }
+
+  win.addEventListener = (event, handler) => {
+      console.log(`DEBUG: window.addEventListener('${event}') called`);
+  };
+
+  return { window: win, document: doc };
+}
+
+test("Debug HashtagStripHelper fallback", (t) => {
+  console.log("Global ResizeObserver type:", typeof globalThis.ResizeObserver);
+
+  const { window, document } = createDom({ useResizeObserver: false });
+  const container = document.getElementById("container");
+
+  const helper = new HashtagStripHelper({ window, document });
+
+  console.log("Helper window ResizeObserver:", typeof helper.window.ResizeObserver);
+
+  helper.mount(container);
+  // Need to update with tags to trigger render and observer setup
+  helper.update([{ tags: [["t", "test"]] }]);
+});
diff --git a/tests/ui/components/hashtagStripHelper.test.mjs b/tests/ui/components/hashtagStripHelper.test.mjs
index deba8a6d..298f28bf 100644
--- a/tests/ui/components/hashtagStripHelper.test.mjs
+++ b/tests/ui/components/hashtagStripHelper.test.mjs
@@ -3,6 +3,9 @@ import assert from "node:assert/strict";
 import { JSDOM } from "jsdom";
 import { HashtagStripHelper } from "../../../js/ui/components/hashtagStripHelper.js";
 
+// Capture original global ResizeObserver in case it was polyfilled by setup scripts
+const originalGlobalResizeObserver = globalThis.ResizeObserver;
+
 function createDom(options = {}) {
   const dom = new JSDOM("<!DOCTYPE html><body><div id='container'></div></body>", {
     pretendToBeVisual: true,
@@ -81,6 +84,12 @@ function createDom(options = {}) {
     win.ResizeObserver = MockResizeObserver;
   } else {
     delete win.ResizeObserver;
+    // Important: HashtagStripHelper falls back to globalThis.ResizeObserver if window.ResizeObserver is missing.
+    // We must ensure globalThis.ResizeObserver is also hidden for the fallback tests.
+    // We rely on the test case to restore it using t.after().
+    if (globalThis.ResizeObserver) {
+        delete globalThis.ResizeObserver;
+    }
   }
 
   // Helper to trigger resize from observer
@@ -160,6 +169,12 @@ test("HashtagStripHelper uses ResizeObserver when available", (t) => {
 });
 
 test("HashtagStripHelper falls back to window resize with RAF", (t) => {
+  t.after(() => {
+    if (originalGlobalResizeObserver) {
+        globalThis.ResizeObserver = originalGlobalResizeObserver;
+    }
+  });
+
   const { window, document } = createDom({
     useResizeObserver: false,
     mockRaf: true,
@@ -196,6 +211,12 @@ test("HashtagStripHelper falls back to window resize with RAF", (t) => {
 });
 
 test("HashtagStripHelper falls back to window resize with setTimeout when RAF is missing", (t) => {
+  t.after(() => {
+      if (originalGlobalResizeObserver) {
+          globalThis.ResizeObserver = originalGlobalResizeObserver;
+      }
+  });
+
   const { window, document } = createDom({
     useResizeObserver: false,
     mockRaf: false, // Ensure RAF is missing/removed