Why are the messages confused?

Question

WPF

I'm writing a chat. I have a ready-made control for displaying messages. It works stably when it lies on the form, but when I try to shove it into the TabControl, it starts to get stupid. Messages from different tabs are confused, all sorts of errors come up periodically, then the tabs are not visible, then the messages ... Tell me please, what is the problem?
The code did not fit here by the number of characters, so here it is: the control itself , the ViewModel, and the markup .