LibWeb: Remove incomplete attempt to avoid repaints outside viewport

This code didn't account for position:fixed elements, which meant that
we'd swallow repaints for such elements when the viewport wasn't
scrolled to the top of the page.

We replace the incomplete optimization with a FIXME noting that this
needs to be handled correctly when reintroduced.

Userland/Libraries/LibWeb/HTML/Navigable.cpp

@@ -1981,8 +1981,8 @@ void Navigable::set_needs_display()
 
 void Navigable::set_needs_display(CSSPixelRect const& rect)
 {
-    if (!viewport_rect().intersects(rect))
-        return;
+    // FIXME: Ignore updates outside the visible viewport rect.
+    //        This requires accounting for fixed-position elements in the input rect, which we don't do yet.
 
     if (is<TraversableNavigable>(*this)) {
         static_cast<TraversableNavigable*>(this)->page().client().page_did_invalidate(to_top_level_rect(rect));