When VE loads we
- Show an empty dummy toolbar
- Then once the code/doc is ready we transition in the real toolbar
The second step only needs to fade in the contents of the toolbar, not the whole toolbar (including the drop shadow), otherwise the drop shadow: appears, disappears then fades in.
This is a lot easier to see if you slow down the transition by running the following in the console:
mw.util.addCSS('.ve-init-mw-desktopArticleTarget-toolbar > .oo-ui-toolbar-bar { transition: opacity 2s }');