CM6 tail: Cell spacing

[vidartf]

Here are screenshots of how a merge looks before and after the CM6 / lab 4 rewrite:

(old version on left, master on right)

The main thing to note is how the vertical spacings have been messed up. These should either be brought in line with the old style, or at least reviewed to be coherent (all things from a single cell should be spaced together).

[krassowski]

Closing as fixed by #728, please reopen if needed.