LibWeb: Find closest scrollable paintable to call handle_mousewheel()

This makes event handler to try to find scrollable paintable in the
chain of containing blocks of hit box. This is very naive and will
have to be improved in the future.

Userland/Libraries/LibWeb/Page/EventHandler.cpp

@@ -163,6 +163,15 @@ bool EventHandler::handle_mousewheel(CSSPixelPoint position, unsigned button, un
     if (auto result = target_for_mouse_position(position); result.has_value())
         paintable = result->paintable;
 
+    auto* containing_block = paintable->containing_block();
+    while (containing_block) {
+        if (containing_block->is_scrollable()) {
+            const_cast<Painting::PaintableBox*>(containing_block->paintable_box())->handle_mousewheel({}, position, buttons, modifiers, wheel_delta_x * scroll_step_size, wheel_delta_y * scroll_step_size);
+            break;
+        }
+        containing_block = containing_block->containing_block();
+    }
+
     if (paintable) {
         paintable->handle_mousewheel({}, position, buttons, modifiers, wheel_delta_x, wheel_delta_y);