Multiple users are reporting (and I confirm) that Merge gadget disappeared on Wikidata when using Vector skin
The problem seems to be with the hidden tabs (which appear from the dropdown menu) and usually include gadgets. This dropdown menu does not appear anymore at all in Vector skin.
Gadgets are still displayed in other skins (e.g. it works in Monobook).
As a result, item merges are too complicated on Wikidata which significantly harms the ability to edit.
The mediawiki core method addPortletLink runs $portlet.removeClass( 'emptyPortlet' );
This should be the responsibility of Vector. Let's move that to its JS.