preview

opening multiple documents in preview in one window and have thumbnails available:

this used to happen automatically until recently it stopped happening.

Now if I try to open multiple documents at once they all open in one window per documents (which is very very tedious to work with).

I have tried preferences but the option is not there.

However if I try to open multiple photos, then the thumbnail is available there...

I don't understand why or how....

I use High Sierra 10.13.6

can anyone help..?