Not possible to place a block by drag & drop above columns
If multiple columns are at the top of a note, it is not possible to drag & drop a paragraph or other block above those columns. It is only possible to move the block on top of one of the columns, but not above all columns.
There also seems to be no way to insert a new block above multiple columns when those columns are at the top of a note. Edit: If you click on the narrow area below the title of the note, then the possibility to insert a new block arises. Unfortunately, this is not obvious, and those who don't know can only find out by bravely trying.
I think, in case of multiple columns there should be a context menu option to add a new block above the columns.
This is true for both the web client and the desktop client (Mac).
Unsuccessful attempt to place a paragraph above these three columns by drag & drop - the expected behavior would be that while dragging the block, a blue indicator would show up that spans all three columns: