When editing preformatted text, hitting Enter ends the preformatted block. Shift-Enter splits the block, unless the cursor is at the end, when it doesn't, for no solid reason.
Oh! I figured the design issue is more complicated:
Currently there are two cases:
1.1 Put the cursor (caret) into already existing, multi-line pre-formatted block.
1.2 Shift+Enter creates line break when cursor is above last line of the preformatted block.
1.3 Shift+Enter makes cursor leave preformatted block when cursor is on the last line of the preformatted block.
2.1 Put cursor in empty space
2.2 Select 'Preformatted' from format dropdown
2.3 Start typing, then press Shift+Enter
2.4 Cursor leaves preformatted block!
So it is impossible to create preformatted blocks from scratch :)
Shift+Enter should just always create a line break.