refactor to improve cell switching in edit mode This code was repeated in both CodeCell and TextCell, both of which are extensions of Cell, so this just unifies the logic in Cell. TextCell had logic here to check if the cell was rendered or not, but I don't believe it is possible to end up triggering such a code path. (Should that be required, I can always just add back these methods to TextCell, performing the .rendered==True check, and calling the Cell prior to this, code mirror at_top would only return true on if the cursor was at the first character of the top line. Now, pressing up arrow on any character on the top line will take you to the cell above. The same applies for the bottom line. Pressing down arrow would only go to the next cell if the cursor was at a location *after* the last character (something that is only possible to achieve in vim mode if the last line is empty, for example). Now, down arrow on any character of the last line will go to the next cell.