Controls in the right panel of a SplitControl are excessively repainted when the splitter is moved.

[gkronber]

Reproduce:

1. Create an ItemCollection<IItem> and add an ItemCollection (several times)
2. Open the view and select all contained items
3. Change the slider of the out-most splitcontainer -> All inner controls are repainted causing flicker and lags.
4. note that resizing the window works smoothly (related #2979)​

[gkronber]

See related #2978