Make VisualEditor a stacking context (#62681)

Co-authored-by: stokesman <presstoke@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: youknowriad <youknowriad@git.wordpress.org>
Co-authored-by: ellatrix <ellatrix@git.wordpress.org>

packages/editor/src/components/visual-editor/style.scss

@@ -3,6 +3,8 @@
 	height: 100%;
 	display: block;
 	background-color: $gray-300;
+	// Make this a stacking context to contain the z-index of children elements.
+	isolation: isolate;
 
 	// Centralize the editor horizontally (flex-direction is column).
 	align-items: center;