Bug: index.ui has VBox with 3 urlwidget subwidgets (top.ui, center.ui, bottom.ui).
When RenderUrlWidget dispatched through ActionDispatcher, loadWidget called
onWidgetLoaded which replaced the ENTIRE root widget. The last urlwidget to
finish loading (bottom.ui) overwrote top.ui and center.ui.
Fix: RenderUrlWidget now maintains local state (loadedWidget) and renders
loaded content in-place. Added ActionDispatcher.loadWidgetInPlace() that loads
via callbacks without calling onWidgetLoaded. Each urlwidget replaces only
itself, preserving sibling widgets in the parent container.
yumoqing
5ba33b9e18