ui: Don't break flex layout when using WithRemSize (#12076)

This PR fixes an issue where the flex hierarchy wasn't getting broken by
the use of `WithRemSize`.

Release Notes:

- N/A

crates/ui/src/with_rem_size.rs

@@ -1,6 +1,6 @@
 use gpui::{
     div, AnyElement, Bounds, Div, DivFrameState, Element, ElementId, GlobalElementId, Hitbox,
-    IntoElement, LayoutId, ParentElement, Pixels, WindowContext,
+    IntoElement, LayoutId, ParentElement, Pixels, StyleRefinement, Styled, WindowContext,
 };
 
 /// An element that sets a particular rem size for its children.
@@ -18,6 +18,12 @@ impl WithRemSize {
     }
 }
 
+impl Styled for WithRemSize {
+    fn style(&mut self) -> &mut StyleRefinement {
+        self.div.style()
+    }
+}
+
 impl ParentElement for WithRemSize {
     fn extend(&mut self, elements: impl IntoIterator<Item = AnyElement>) {
         self.div.extend(elements)