the width of splitview's child should be 'availableWidth', but not 'width'

the width of splitview(column mode)'s child (columnLayout) is always splitview's width, if padding is 0, no problem.

but if I set padding of splitview, the child width still be splitview's width, and render over the splitview, even I bind the child's width to splitview's availablewidth.

so how to make the child's width as the splitview's availablewidth?