FIX: prevents scroll-lock to crash the app (#36246)

jjaffeux · web-flow · commit bc36f0355d3f · 2025-11-26T11:14:57.000+01:00
In rare cases, especially on mobile, we could end up in a situation
where `null` is given to `unlockScroll`, we were given a default arg to
the function through: `unlockScroll(element =
document.scrollingElement)` but this is working only if the given
element is `undefined` and not `null` resulting sometimes in the
following error:

```js
TypeError: null is not an object (evaluating 'element.classList')
```

Following this error, the whole UI would become unresponsive.

Here is a simple example to demonstrate the buggy behavior here:

```js
function greet(name = "World") {
  console.log(`Hello, ${name}!`);
}

greet(); // Hello, World! &#10003; (default applied)
greet(undefined); // Hello, World! &#10003; (default applied)
greet(null); // Hello, null!  &#10007; (default NOT applied!)
```
diff --git a/frontend/discourse/app/lib/scroll-lock.js b/frontend/discourse/app/lib/scroll-lock.js
@@ -2,7 +2,12 @@
    lock scroll of an element using overflow:hidden
    preserve gutter with scroll detection
 */
-function lockScroll(element = document.scrollingElement) {
+function lockScroll(element) {
+  element ??= document.scrollingElement;
+  if (!element) {
+    return;
+  }
+
   let scrollGap = 0;
 
   //Add scroll gap if using default scrolling element
@@ -15,7 +20,12 @@ function lockScroll(element = document.scrollingElement) {
   element.classList.add("scroll-lock");
 }
 
-function unlockScroll(element = document.scrollingElement) {
+function unlockScroll(element) {
+  element ??= document.scrollingElement;
+  if (!element) {
+    return;
+  }
+
   element.classList.remove("scroll-lock");
   element.style.setProperty("--scroll-gap", null);
 }
