On CodeEditor the old toolbar is visible while loading. It gets hidden when the new WikiEditor toolbar is loaded.

WikiEditor hides the old toolbar while loading by

/* Hide the old toolbar until the wikiEditor toolbar is deemed not supported,
 * hide the WikiEditor toolbar until it's css has loaded */
.wikiEditor-oldToolbar,
.wikiEditor-ui-toolbar {
	display: none;
}

in file modules/ext.wikiEditor.toolbar.styles.less. This file is loaded by module ext.wikiEditor.toolbar.styles on position top. CodeEditor does not load this module. This also generates T102738.