Yes, now I get what Quicken is doing. It's humbling to realize I used Quicken for so long without noticing the importance of this interface design. I guess it shows how well developed and refined third-party software can be. It's not particularly noticeable until you stop and think about what's going on.
Quicken includes
both expense categories and
inter-account transfers here in the list.
Note that they are all clearly labelled by type for that reason. "Expense" is one type of transaction; "Transfer" is also a type of transaction. That does not mean they are both categories of expense, which is where we went off track. Categories remain a subset of Expenses. Transfers are not the same thing.
In their interface, Quicken comingles Expenses and Transfers for ease of data entry.
That masks, unfortunately, the fact that when you record an expense in an account, that outgoing amount goes, well, outside to a vendor. The money is deducted from your account and reduces your overall balance.
When you record a transfer, that amount has to go into a different account inside the system to maintain the integrity of the action. The money transferred does not reduce your overall balance.
Given that, I can see a combo box which uses a union query of categories of expense and income along with internal inter-account transfers, using the square bracket convention or one of your choosing to differentiate.
The after update event of the combo box would have to decide which rules to apply to the selection. I.e. if it's an expense or income category, deduct it. If it's a transfer out, deduct it, but also add the same amount to the selected account. If it's a transfer in, add it, but also deduct the same amount from the selected account.
My apologies for being so slow to catch on.