Thanks for your report. Funnily this bug existed for almost 6 years, without anyone reporting it. I've spotted and fixed it few weeks back:
https://github.com/winscp/winscp/commit/c4a15d4a314cf8a48a23642305bf6de61f6b74bb
And now you report it :)
Adding it to the tracker now: https://winscp.net/tracker/1908

WinSCP: 5.17.7 build 10640

I used to have a "diff" command in the Synchronize checklist -> Commands -> Diff, but it's not there anymore. I updated from a version I don't remember (I just overwrote the winscp.exe). However I can't seem to be able to add it there either because:

Synchronize checklist -> Commands -> Customize... button does nothing

See the attached image of the button I press. I'd like to add the diff command back there, but since the customize... button does not work I can't find a way.

Thanks.