UX: Make sticky avatars behavior consistent (#27223)

Previously, avatars would be 'sticky' when:

1. The post was longer than the viewport

OR

2. You were scrolling up

The difference in behavior based on scroll direction doesn't 'feel' quite right. This commit makes the behavior consistent, so sticky avatar logic is applied to all posts regardless of scroll direction.

app/assets/javascripts/discourse/app/modifiers/sticky-avatars.js

@@ -122,14 +122,7 @@ export default class StickyAvatars extends Modifier {
               return;
             }
 
-            const postContentHeight =
-              entry.target.querySelector(".contents")?.clientHeight;
-            if (
-              this.direction === "⬆️" ||
-              postContentHeight > window.innerHeight - offset
-            ) {
-              entry.target.classList.add(STICKY_CLASS);
-            }
+            entry.target.classList.add(STICKY_CLASS);
           });
         },
         {