When a transaction is imbalanced, for example when copying an earlier
transaction and changing one of the amounts, one often wants to apply the
imbalance to one of the other existing splits.

The best ways currently available to do this are cumbersome: 

A/ blank out the split, see the imbalance update on the last line, and copy the
imbalance to the current split; or 
B/ edit the value of the current split to use the built-in calculator to add the
current imbalance to it; or
C/ copy the current imbalance to the blank side of the current split (CR or DR)
with the appropriate sign to have the net value calculated.

All of these methods require more typing and more thinking than should be
necessary for a common operation.  I suggest considering implementing these
interfaces:

A/ Make the "imbalance hatching" (the small X in a square that appears when the
transaction is imbalanced) active.  Clicking on the X would apply the imbalance
to the selected split; and

B/ Have some keystroke, for example "!" apply the imbalance to the current
split.  This might be part of the built-in calculator of the amount fields.