Fix #6081

Test code

import flet as ft


def main(page: ft.Page):
    def handle_size_change(e: ft.LayoutSizeChangeEvent[ft.Container]):
        e.control.content.value = f"{int(e.width)} x {int(e.height)}"

    page.add(
        ft.Container(
            expand=True,
            alignment=ft.Alignment.CENTER,
            bgcolor=ft.Colors.BLUE_ACCENT,
            content=ft.Text(color=ft.Colors.WHITE, weight=ft.FontWeight.BOLD),
            size_change_interval=100,
            on_size_change=handle_size_change,
        )
    )


if __name__ == "__main__":
    ft.run(main)

Summary by Sourcery

Add an opt-in layout size change event to layout-based controls and migrate dependent widgets to use the unified LayoutControl wrapper.

New Features:

• Introduce LayoutSizeChangeEvent and on_size_change callback on LayoutControl to notify when a control's rendered size changes.
• Add size_change_interval property to control how frequently size change events are fired.

Enhancements:

• Wrap layout children in a SizeChangeObserver/MeasureSize layer when size change events are enabled, throttling notifications to reduce layout overhead.
• Replace the legacy ConstrainedControl wrapper with LayoutControl across video, charts, ads, map, webview, and related Flutter integrations.
• Refine various control and gesture documentation examples for clarity and consistency, including container and reorderable list view docs.

Documentation:

• Document size-aware container usage and add a corresponding screenshot to the container control docs, while simplifying other container doc images.

Tests:

• Add screenshot-based integration tests for size-aware container and nested themes container examples.