now I have drilled down the behavior to source locations:
1) The Widths of the RadSplitContainers on the left and right are set fix either by the ResizeControl on drag drop or DockingPanel.MeasureOverride with initial widths defined by the user.
2) On application window resize RadDockPanel tries to Arrange both split containers with remaining size.
-> First conclusion: RadDockPanel does not support proporional arrange if the available space < desired space, because it works a little bit like DockPanel. It gives the space which is available and not consumed by previously arranged children to its children and it does nothing to try to fit all children if not enough space is available.
3) When RadDockPanel trys to arrange the RadSplitContainers with remaining size it does not work as expected, because RadSplitContainers do not have ArrangeOverrides, so FrameworkElement.ArrangeOverride is called, which clipps the content when available width is less than the set Width (see Point 1)
-> Second conclusion: So even if proportional arrange would be realized in RadDockPanel, an ArrangeOverride would have to be added to RadSplitContainer, which would arrange the split container to the available size so that clipping woulf not occur.
Seems like a feature request.