I used to arrange two panels under a visual element by flex-grow: 1, and set the width, e.g. to 30% and 70% to control relative size.
Now, the TwoPaneSplitView add the dragging for resize, but the fix pane’s initial width/height is an attribute(float) that seems set only how many pixels. Is their ways to init the width in % of parent container size.
Thanks