Add a sticky issue header to issue page (#42684).

Patch by Mizuki ISHIKAWA (user:ishikawa999).


git-svn-id: https://svn.redmine.org/redmine/trunk@23752 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/javascript/controllers/sticky_issue_header_controller.js

@@ -0,0 +1,22 @@
+import { Controller } from "@hotwired/stimulus";
+
+export default class extends Controller {
+  static targets = ["original", "stickyHeader"];
+
+  connect() {
+    if (!this.originalTarget || !this.stickyHeaderTarget) return;
+
+    this.observer = new IntersectionObserver(
+      ([entry]) => {
+        this.stickyHeaderTarget.classList.toggle("is-visible", !entry.isIntersecting);
+      },
+      { threshold: 0 }
+    );
+
+    this.observer.observe(this.originalTarget);
+  }
+
+  disconnect() {
+    this.observer?.disconnect();
+  }
+}