it breaks the logic of slotIdentityChanged if we update mId directly.

no sticky identities anymore -> so update the comments

this todo doesn't makes sense anymore, or at least I do not understand what block should be moved

don't use auto for uint please (for each simple type).
We will have a lot of "auto" in source code and it's not easy after that to read it

Do you have a test case for it ?

Nope.

The testcase in D892 is actuatually using the X-KMail-Identity header to give the information what identity should be used to kmcomposewin. But kmcomposerwin is initalized with the default identity. And with that a slotIdentityChanged is executed (see line 1555/1545 old/new). That's why I looked inside slotIdentityChanged and see that this function compare what different settings the old and new identitiy have. And only triggers something if there is a difference. So if we set mId before the diff is not available.

-> so there is a test case that scrates the sureface of slotIdentityChanged.

So you need to move it in 891
It's not same patch as cleanup