fix: ensure relative z-index of elements added to frame is retained (#7134)

dev-docs/docs/codebase/frames.mdx

@@ -0,0 +1,22 @@
+# Frames
+
+## Ordering
+
+Frames should be ordered where frame children come first, followed by the frame element itself:
+
+```
+[
+  other_element,
+  frame1_child1,
+  frame1_child2,
+  frame1,
+  other_element,
+  frame2_child1,
+  frame2_child2,
+  frame2,
+  other_element,
+  ...
+]
+```
+
+If not oredered correctly, the editor will still function, but the elements may not be rendered and clipped correctly. Further, the renderer relies on this ordering for performance optimizations.