Well, got home and did some further investigation.
It seems there's something else going on.
Upon loading just the navigation page enough room is freed up to allow space for the link to the item gone missing (in my case sysmon).
And, when I downsize the whole browser menu scrollbars do appear, so with the scrolling nothing is wrong.
Silly me.
But why did sysmon disappear ?
It was there, but after installing another contrib (I think it was the printerqueue admin) it disappeared.
Well, now at least I know what to do on a rainy night with nothing on the tele
If any of you guys should have a clue you'll let me know, won't you ?