For anyone whom thought that rexUI seems not working

Phaser 3
1

It was bothering me that adding children to sizer does not reflected directly on the canvas.
Then I found this fact; whenever you change the sizer of a game object included in the rexUI, you need to redraw by layout().

For example;

// first, we add a rexUI gameobject.
var a = new CustomScrollablePanel(x, y, width, height) // extended ScrollablePanel of rexUI
// if we add or remove a child to the panel
a.getElement('panel').add(newSizer);
// then we need to redraw the gameobject
a.layout();

layout() only reflects the children when called.