When there are conflicting changes in other branches (i.e. master) Stash stops you from pressing the "merge" button. The next to the merge button tells you what command are required to resolve the conflicts before merging.
After you follow the instructions, you can see the merge commit (after switching tabs between 'dff' and 'commits'.
The merge button is inoperable and the is still present next to it.
Only after refreshing the whole page the button is enabled and the is removed.
Version: v3.3.0-m14