diff --git a/client/src/components/WidgetContainer.jsx b/client/src/components/WidgetContainer.jsx
index bb088388..38c2f1bf 100644
--- a/client/src/components/WidgetContainer.jsx
+++ b/client/src/components/WidgetContainer.jsx
@@ -19,6 +19,7 @@ import { canCommitResize } from '../utils/resizeGuard';
 import { shouldAcceptLayoutChange, reconcileLayoutWithLive } from '../utils/layoutSync';
 import { buildLayout, savedSourcesById } from '../utils/gridPlacement';
 import { resizeButtonMetrics, widgetPixelSize } from '../utils/resizeButtons.js';
+import { createContainerWidthWatcher, gridColsForWidth } from '../utils/containerWidth.js';
 import { readGridMetrics } from '../utils/gridMetrics';
 import { frameDecoration } from '../utils/widgetFrame';
 import FrameOrnaments from './FrameOrnaments';
@@ -198,31 +199,25 @@ const WidgetContainer = ({
     }, 500);
   }, []);
 
-  // Update container width and grid columns based on screen size
+  // The container width drives the grid, so re-measure whenever the
+  // container element itself changes size — not just on window resize.
+  // In mirror the canvas starts at the 1280 fallback width and widens to
+  // the target's real width once its viewport arrives; that widening is
+  // not a window resize, so a window-only listener leaves the grid laid
+  // out at the fallback size (the weather widget then lands compact at
+  // 294px instead of medium at 332px). The watcher reports only on an
+  // actual width change, so no layout rebuild runs on a no-op.
   useEffect(() => {
-    const updateDimensions = () => {
-      if (containerRef.current) {
-        const computed = window.getComputedStyle(containerRef.current);
-        const paddingLeft = parseFloat(computed.paddingLeft) || 0;
-        const paddingRight = parseFloat(computed.paddingRight) || 0;
-        const width = Math.max(0, containerRef.current.clientWidth - paddingLeft - paddingRight);
+    const element = containerRef.current;
+    if (!element) return undefined;
+    const watcher = createContainerWidthWatcher({
+      element,
+      onChange: (width) => {
         setContainerWidth(width);
-
-        // Responsive grid columns
-        if (width < 600) {
-          setGridCols(4); // Mobile: 4 columns
-        } else if (width < 960) {
-          setGridCols(8); // Tablet: 8 columns
-        } else {
-          setGridCols(12); // Desktop: 12 columns
-        }
-      }
-    };
-
-    updateDimensions();
-    window.addEventListener('resize', updateDimensions);
-
-    return () => window.removeEventListener('resize', updateDimensions);
+        setGridCols(gridColsForWidth(width));
+      },
+    });
+    return () => watcher.disconnect();
   }, []);
 
   useEffect(() => {
diff --git a/client/src/utils/containerWidth.js b/client/src/utils/containerWidth.js
new file mode 100644
index 00000000..386cecd1
--- /dev/null
+++ b/client/src/utils/containerWidth.js
@@ -0,0 +1,68 @@
+// Container-width measurement for the dashboard grid (WidgetContainer).
+//
+// The grid's width is the container element's own width, so it has to be
+// re-measured whenever that element changes size — not just on window
+// resize. In mirror the canvas starts at the 1280 fallback width and
+// widens to the target's real width once its viewport arrives; that
+// widening is not a window resize, so a window-only listener leaves the
+// grid laid out at the fallback size (the weather widget then lands
+// compact at 294px instead of medium at 332px).
+//
+// The watcher reports only when the measured width actually changes, so a
+// resize storm or a state update that leaves the width alone never
+// re-runs the layout rebuild. The container is width:100% of its parent,
+// so changing the grid's width prop never changes the container's own
+// width — the observer cannot feed itself.
+
+export function measureContainerWidth(element) {
+  if (!element || typeof window === 'undefined') return null;
+  const computed = window.getComputedStyle(element);
+  const paddingLeft = parseFloat(computed.paddingLeft) || 0;
+  const paddingRight = parseFloat(computed.paddingRight) || 0;
+  return Math.max(0, element.clientWidth - paddingLeft - paddingRight);
+}
+
+// Responsive grid columns, matching the breakpoints WidgetContainer has
+// always used.
+export function gridColsForWidth(width) {
+  if (width < 600) return 4; // Mobile: 4 columns
+  if (width < 960) return 8; // Tablet: 8 columns
+  return 12; // Desktop: 12 columns
+}
+
+export function createContainerWidthWatcher({ element, onChange }) {
+  let lastWidth = null;
+  let observer = null;
+
+  const check = () => {
+    const width = measureContainerWidth(element);
+    if (width === null || width === lastWidth) return;
+    lastWidth = width;
+    onChange(width);
+  };
+
+  // Initial measure, so the grid is right on first paint even if the
+  // observer never fires.
+  check();
+
+  if (typeof ResizeObserver !== 'undefined' && element) {
+    observer = new ResizeObserver(check);
+    observer.observe(element);
+  }
+
+  // Window resize stays as a fallback for environments without
+  // ResizeObserver; the change guard above makes the duplicate harmless
+  // where both fire.
+  if (typeof window !== 'undefined') {
+    window.addEventListener('resize', check);
+  }
+
+  return {
+    disconnect() {
+      if (observer) observer.disconnect();
+      if (typeof window !== 'undefined') {
+        window.removeEventListener('resize', check);
+      }
+    },
+  };
+}
diff --git a/client/src/utils/containerWidth.test.js b/client/src/utils/containerWidth.test.js
new file mode 100644
index 00000000..3a986baf
--- /dev/null
+++ b/client/src/utils/containerWidth.test.js
@@ -0,0 +1,127 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import {
+  measureContainerWidth,
+  gridColsForWidth,
+  createContainerWidthWatcher,
+} from './containerWidth.js';
+
+function fakeElement({ clientWidth, paddingLeft = '16px', paddingRight = '16px' } = {}) {
+  return { clientWidth, __paddingLeft: paddingLeft, __paddingRight: paddingRight };
+}
+
+describe('measureContainerWidth', () => {
+  beforeEach(() => {
+    vi.stubGlobal('window', {
+      getComputedStyle: (el) => ({
+        paddingLeft: el.__paddingLeft,
+        paddingRight: el.__paddingRight,
+      }),
+      addEventListener: vi.fn(),
+      removeEventListener: vi.fn(),
+    });
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it('subtracts horizontal padding from clientWidth', () => {
+    expect(measureContainerWidth(fakeElement({ clientWidth: 1440 }))).toBe(1408);
+    expect(measureContainerWidth(fakeElement({ clientWidth: 1280 }))).toBe(1248);
+  });
+
+  it('floors at 0 and returns null without an element', () => {
+    expect(measureContainerWidth(fakeElement({ clientWidth: 10 }))).toBe(0);
+    expect(measureContainerWidth(null)).toBe(null);
+    expect(measureContainerWidth(undefined)).toBe(null);
+  });
+});
+
+describe('gridColsForWidth', () => {
+  it('uses 4 / 8 / 12 columns at the 600 / 960 breakpoints', () => {
+    expect(gridColsForWidth(0)).toBe(4);
+    expect(gridColsForWidth(599)).toBe(4);
+    expect(gridColsForWidth(600)).toBe(8);
+    expect(gridColsForWidth(959)).toBe(8);
+    expect(gridColsForWidth(960)).toBe(12);
+    expect(gridColsForWidth(1408)).toBe(12);
+  });
+});
+
+describe('createContainerWidthWatcher', () => {
+  let element;
+  let observerInstances;
+  let resizeListeners;
+
+  beforeEach(() => {
+    element = fakeElement({ clientWidth: 1280 });
+    observerInstances = [];
+    resizeListeners = [];
+    vi.stubGlobal('window', {
+      getComputedStyle: (el) => ({
+        paddingLeft: el.__paddingLeft,
+        paddingRight: el.__paddingRight,
+      }),
+      addEventListener: (type, fn) => { resizeListeners.push([type, fn]); },
+      removeEventListener: (type, fn) => {
+        resizeListeners = resizeListeners.filter(([t, f]) => !(t === type && f === fn));
+      },
+    });
+    vi.stubGlobal('ResizeObserver', class {
+      constructor(cb) { this.cb = cb; observerInstances.push(this); }
+      observe() {}
+      disconnect() { this.disconnected = true; }
+    });
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it('reports the initial width immediately', () => {
+    const onChange = vi.fn();
+    const watcher = createContainerWidthWatcher({ element, onChange });
+    expect(onChange).toHaveBeenCalledTimes(1);
+    expect(onChange).toHaveBeenCalledWith(1248);
+    watcher.disconnect();
+  });
+
+  it('reports a width change from the observer once, and ignores repeats at the same width', () => {
+    const onChange = vi.fn();
+    const watcher = createContainerWidthWatcher({ element, onChange });
+    expect(observerInstances).toHaveLength(1);
+
+    // Mirror viewport arrives: canvas widens 1280 -> 1440.
+    element.clientWidth = 1440;
+    observerInstances[0].cb();
+    expect(onChange).toHaveBeenCalledTimes(2);
+    expect(onChange).toHaveBeenLastCalledWith(1408);
+
+    // Same width again: no further report, so no layout rebuild.
+    observerInstances[0].cb();
+    expect(onChange).toHaveBeenCalledTimes(2);
+    watcher.disconnect();
+  });
+
+  it('falls back to the window resize listener without ResizeObserver', () => {
+    vi.stubGlobal('ResizeObserver', undefined);
+    const onChange = vi.fn();
+    const watcher = createContainerWidthWatcher({ element, onChange });
+    expect(onChange).toHaveBeenCalledTimes(1);
+
+    element.clientWidth = 1440;
+    const resize = resizeListeners.find(([type]) => type === 'resize')[1];
+    resize();
+    expect(onChange).toHaveBeenCalledTimes(2);
+    expect(onChange).toHaveBeenLastCalledWith(1408);
+    watcher.disconnect();
+  });
+
+  it('disconnect stops the observer and the resize listener', () => {
+    const onChange = vi.fn();
+    const watcher = createContainerWidthWatcher({ element, onChange });
+    watcher.disconnect();
+    expect(observerInstances[0].disconnected).toBe(true);
+    expect(resizeListeners).toHaveLength(0);
+  });
+});