Multi-currency transactions with trading accounts from 2.4.13 getting messed up in latest 2.6.3

[Mark]

I think I figured out what is happening (not why, and also no fix for 
it, though):

For some reason, when I created the multi-currency transactions in 
2.4.13, GnuCash did not create any exchange rate record in the prices 
repository. This shouldn't be a problem, since the exchange rate is 
implicitly stored in the split lines of the transaction.
And so, GnuCash 2.4.13 is fine with this.

For some reason, version 2.6.3 is misbehaving when it finds transactions 
without a matching exchange rate.
Instead of just not touching the transaction and its split lines, it 
assumes an exchange rate of 1 and then based on that exchange rate, 
modifies one of the 2 trading account split amounts (this is the crucial 
thing which definitely must NOT happen), thus imbalancing (and 
essentially corrupting) the transaction. This is not happening in the 
file, but in memory.
And because it just imbalanced the transaction by messing with the 
Trading account split amount, it adds the Imbalance split line to 
compensate.

When I try to fix the error by reverting the 4 lines back to their 
original amounts and removing the Imbalance split line, as soon as I try 
to save the transaction, it once again tries to pull the exchange rate, 
does not find it and kicks off the previously described corrupting 
mechanism.

The only way to fix the corrupted transactions is to use "Edit exchange 
rate" on the transaction, which can not be regarded as a fix for this 
bug, though.

I also documented this in my bug submission.


I wonder if there are other circumstances, in which existing transations 
are modified in the background without the user knowing...
Fortunately, this one was pretty obvious, so that I stumbled over it 
right away, but others might b more subtle...

Mark