The problem is because the DropDownOpened event is asynchronous, e.g. it is raised in a Dispatcher.BeginInvoce callback. I will check whether this is needed anymore, I remember fixing a problem in an older version this way (I know, it is a bad thing to use Dispatcher), which might be irrelevant now. To workaround the problem I would recommend using a flag:
- when you populate the combo you rise the flag, then open the dropdown
- when the dropdown opens, if the flag is raised, do nothing, but lower the flag, otherwise execute the code you need.
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