Hello Buks,

Thank you for the provided information. 

Indeed the described behavior is reproducible in .net versions higher than 4.5. That said, I do hope that one of the suggested workarounds will be suitable for your scenario.

Regards,
Vladimir Stoyanov
Progress Telerik

Another possible solution would be to set the FrameworkCompatibilityPreferences.KeepTextBoxDisplaySynchronizedWithTextProperty to false. More information can be found here: https://msdn.microsoft.com/en-us/library/system.windows.frameworkcompatibilitypreferences.keeptextboxdisplaysynchronizedwithtextproperty(v=vs.110).aspx

This behaviour is reproducible only in .Net 4.5. It can be observed with a TextBox outside of RadDataForm if its Binding.UpdateSourceTrigger property is set to PropertyChanged.  As it turns out, this behaviour is the expected one since Microsoft had introduced it as a fix for an issue in .Net 4.0.  You can check the following article:

https://connect.microsoft.com/VisualStudio/feedback/details/737301/binding-to-wpf-textbox-seems-to-be-broken-in-net-4-5-beta

 for a reference. As Microsoft had stated: If you're targeting 4.5+ and you really want UpdateSourceTrigger=PropertyChanged, consider also setting Delay=300. This property (new in 4.5) causes the binding to wait 300ms before committing the edit, giving the user time to type a digit after the decimal point. This also delays the validation feedback until the user has been idle for 300ms - you may or may not like this. [The value 300 is just what I've found to be comfortable; you can adjust it to your own taste.]