# ResizeObserver Sizing

> no gray tiles in collapsible layouts.

---

.. llms_copy::ResizeObserver Sizing

.. toc::

### Overview

This page demonstrates ResizeObserver Sizing.

### Live demo

.. exec::docs.resize-observer.example
    :code: false

### Source


```python
# File: docs/resize-observer/example.py

"""ResizeObserver Sizing — no gray tiles in collapsible layouts."""

import dash_mantine_components as dmc
from dash import Input, Output, clientside_callback, html
from dl2_shared import code_panel, header, info_panel

JS = """// Leaflet 2 observes its container with a ResizeObserver (trackResize,
// default ON). When the container changes size — opening a side panel, a tab,
// an accordion — the map re-renders itself. No more:
//     map.invalidateSize();   // the classic Leaflet 1 dance
const map = new leaflet.Map(el).setView([29.7589, -95.3677], 12);"""

component = dmc.Stack(
    [
        header(
            "ResizeObserver Sizing",
            "Leaflet 1's classic pain: a map inside a hidden tab or collapsible panel "
            "renders gray until you manually call invalidateSize(). v2 watches its "
            "container with a ResizeObserver and fixes itself. Toggle the side panel — "
            "the map below resizes cleanly, with zero invalidateSize() calls.",
            badge="no invalidateSize",
        ),
        dmc.Button(
            "Toggle side panel", id="resize-toggle", color="green", variant="light"
        ),
        html.Div(
            id="resize-row",
            className="dl2-resize-row",
            children=[
                html.Div(className="leaflet2-map", **{"data-demo": "resize-observer"}),
                html.Div(
                    className="dl2-resize-panel",
                    children=[
                        dmc.Title("Side panel", order=5),
                        dmc.Text(
                            "Opening/closing me changes the map's width. Watch it reflow "
                            "instantly — no gray tiles, no manual resize call.",
                            size="sm",
                            c="dimmed",
                        ),
                    ],
                ),
            ],
        ),
        code_panel("It just works", JS),
        info_panel(
            "Why this matters in Dash",
            dmc.Text(
                "Dash layouts lean on tabs, accordions and AppShell panels that hide/resize "
                "content. With dash-leaflet (Leaflet 1) you wire invalidateSize on every "
                "visibility change. With v2 it's automatic.",
                size="sm",
            ),
        ),
    ],
    gap="md",
)

# Toggle the .open class on the flex row; Leaflet's ResizeObserver does the rest.
clientside_callback(
    """
    function(n) {
        var r = document.getElementById('resize-row');
        if (r) { r.classList.toggle('open'); }
        return window.dash_clientside.no_update;
    }
    """,
    Output("resize-toggle", "id"),
    Input("resize-toggle", "n_clicks"),
    prevent_initial_call=True,
)
```

    :defaultExpanded: false
    :withExpandedButton: true


---

*Source: /resize-observer*