Previously, we were setting tab actions only for the active tab on a tab
change, and the same actions for the previous tab were removed.
Unfortunately, this also happened when making a new tab, which meant
that you could trick the cell editor to jump to the new sheet and start
writing there.
To fix this, every view will always have on_selection_changed
and on_selection_dropped assigned. I haven't seen much difference in
the memory usage, so I guess it'll be fine :)
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