So this is one of the strangest bugs I have run into and quite a hard repro to find, but it is consistent and I have found a workaround.

If I start my VR app without first starting SteamVR, I can not click on TabItem headers. The TabControl items and headers are visible and TabItem content can be clicked and I can change the tab through code but can't click the tabs.

I raycast to find where on the UI I hit and send the click with MouseButtonDown((int)f_x, (int)f_y, f_button);

Everything else (and as I said including the TabItem content is responding as expected, but the actual tabs can not be clicked.

If I place a button on the page with this codebehind and click it the problem goes away:
private void Test_Button_Click(object sender, RoutedEventArgs e)
{
x_tabControl.View.Activate();
}

I'm setting this to major since starting SteamVR through my app is the standard behavior for most cases.