Define z-index property for the mw-json-schema-code-samples CSS class to ensure proper stacking

Said class is introduced and used by the EventLogging extension.

Bug: T249557
Change-Id: I5f45d4e5cac9b614f26b17f2507e1f72fc39035b

resources/extensions/EventLogging.jsonSchema.less

@@ -0,0 +1,4 @@
+/* Define z-index property to ensure proper stacking, thus fixing T249557 */
+.mw-json-schema-code-samples {
+	z-index: 1;
+}

skin.json

@@ -97,6 +97,7 @@
 		"timeless": {
 			"+oojs-ui-core.styles": "resources/ooui.less",
 			"+ext.echo.styles.badge": "resources/extensions/Echo.less",
+			"+ext.eventLogging.jsonSchema.styles": "resources/extensions/EventLogging.jsonSchema.less",
 			"+ext.visualEditor.core": "resources/extensions/VisualEditor.core.less",
 			"+ext.visualEditor.desktopArticleTarget.noscript": "resources/extensions/VisualEditor.article.less",
 			"+ext.CodeMirror.visualEditor.init": "resources/extensions/VisualEditor.CodeMirror.less",