Improving selection at Editing side.
The setSelection algorithm should take Regions into account when traversing the RenderTree.
Set SelectionNone in Regions when clearing the selection.
Set SelectionStart/SelectionEnd to the Region blocks (if necessary)
repaintSeleciton should take Regions into account when traversing the RenderTree.
Improving the block gaps filling.
Set SelectionInside flag to the RenderRegion blocks.
Avoid fill the vertical gap for RenderNamedFlowFragment