diff --git a/apps/web/src/viewmodels/structures/auto-collapse/behaviours/CollapseOnWindowResizeBehaviour.ts b/apps/web/src/viewmodels/structures/auto-collapse/behaviours/CollapseOnWindowResizeBehaviour.ts
new file mode 100644
index 0000000000..c2a4b57e2e
--- /dev/null
+++ b/apps/web/src/viewmodels/structures/auto-collapse/behaviours/CollapseOnWindowResizeBehaviour.ts
@@ -0,0 +1,97 @@
+/*
+ * Copyright 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+import { throttle } from "lodash";
+
+import UIStore, { UI_EVENTS } from "../../../../stores/UIStore";
+import { BaseCollapseBehaviour } from "./BaseCollapseBehaviour";
+import type { CollapseHandler } from "../CollapseHandler";
+
+/**
+ * The viewport width below which the left panel will be auto-collapsed.
+ */
+const AUTO_COLLAPSE_WIDTH = 768;
+
+/**
+ * Implements auto-collapse logic that collapses and expands the left panel when the
+ * app window is resized.
+ */
+export class CollapseOnWindowResizeBehaviour extends BaseCollapseBehaviour {
+    /**
+     * If this boolean is true, we won't auto collapse the panel when the
+     * window is resized to be smaller than AUTO_COLLAPSE_WIDTH.
+     *
+     * If the panel is auto-collapsed and then the user manually expands the
+     * panel, we want to make sure that further window resizing does not collapse
+     * the panel.
+     */
+    private disableAutoCollapse = false;
+
+    public constructor(collapseHandler: CollapseHandler) {
+        super(collapseHandler);
+        UIStore.instance.on(UI_EVENTS.WidthIncreased, this.onWindowWidthIncreased);
+        UIStore.instance.on(UI_EVENTS.WidthDecreased, this.onWindowWidthDecreased);
+    }
+
+    private onWindowWidthDecreased = throttle((currentWindowWidth: number): void => {
+        // If the panel is already collapsed, we have nothing else left to do.
+        if (this.collapseHandler.isAutoCollapsed || this.collapseHandler.panelHandle?.isCollapsed()) return;
+
+        // We were already auto-collapsed and the user has manually resized the panel.
+        // Don't auto-collapse again.
+        if (this.disableAutoCollapse) return;
+
+        if (currentWindowWidth <= AUTO_COLLAPSE_WIDTH) {
+            this.collapseHandler.collapse();
+            console.log("\t collapsed panel");
+        }
+    }, 50);
+
+    public onLeftPanelResized = (): void => {
+        if (this.collapseHandler.isAutoCollapsed) {
+            // Track that the user has manually resized the auto-collapsed panel.
+            this.disableAutoCollapse = true;
+        }
+    };
+
+    private onWindowWidthIncreased = throttle((currentWindowWidth: number): void => {
+        if (currentWindowWidth > AUTO_COLLAPSE_WIDTH) {
+            // Reset the flag when we cross the collapse width boundary.
+            this.disableAutoCollapse = false;
+            // If the panel isn't already collapsed, we don't need to expand the panel.
+            if (!this.collapseHandler.isAutoCollapsed) return;
+            // As the window is resized, react-resizable-panels is also resizing the panels.
+            // We'll expand the panel after a second to avoid racing with the library logic.
+            window.setTimeout(() => {
+                // this.disableAutoCollapse = false;
+                this.collapseHandler.expand();
+            }, 1000);
+        }
+    }, 50);
+
+    /**
+     * Whether the window is currently being resized.
+     */
+    public get shouldIgnoreResize(): boolean {
+        // When the window is resized, the panel is resized in various ways.
+        // These transient changes should not be persisted in settings.
+        // So early return if that is the case.
+        return UIStore.instance.isWindowBeingResized;
+    }
+
+    /**
+     * Remove's any event listeners used by this class.
+     */
+    public dispose = (): void => {
+        UIStore.instance.off(UI_EVENTS.WidthIncreased, this.onWindowWidthIncreased);
+        UIStore.instance.off(UI_EVENTS.WidthDecreased, this.onWindowWidthDecreased);
+    };
+
+    public static shouldStartCollapsed(): boolean {
+        return UIStore.instance.windowWidth <= AUTO_COLLAPSE_WIDTH;
+    }
+}
diff --git a/apps/web/test/viewmodels/structures/auto-collapse/behaviours/CollapseOnWindowResizeBehaviour-test.ts b/apps/web/test/viewmodels/structures/auto-collapse/behaviours/CollapseOnWindowResizeBehaviour-test.ts
new file mode 100644
index 0000000000..7d096c4497
--- /dev/null
+++ b/apps/web/test/viewmodels/structures/auto-collapse/behaviours/CollapseOnWindowResizeBehaviour-test.ts
@@ -0,0 +1,63 @@
+/*
+ * Copyright 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+import UIStore, { UI_EVENTS } from "../../../../../src/stores/UIStore";
+import { CollapseOnWindowResizeBehaviour } from "../../../../../src/viewmodels/structures/auto-collapse/behaviours/CollapseOnWindowResizeBehaviour";
+import type { CollapseHandler } from "../../../../../src/viewmodels/structures/auto-collapse/CollapseHandler";
+import { MockCollapseHandler } from "../mocks";
+
+jest.useFakeTimers();
+
+describe("CollapseOnWindowResizeBehaviour", () => {
+    it("Should collapse/expand the panel when the window is resized", () => {
+        const collapseHandler = new MockCollapseHandler() as unknown as CollapseHandler;
+        new CollapseOnWindowResizeBehaviour(collapseHandler);
+        // Making the window smaller should collapse the panel.
+        UIStore.instance.emit(UI_EVENTS.WidthDecreased, 750);
+        expect(collapseHandler.collapse).toHaveBeenCalledTimes(1);
+        // Making the window larger should expand the panel.
+        UIStore.instance.emit(UI_EVENTS.WidthIncreased, 950);
+        jest.runAllTimers();
+        expect(collapseHandler.expand).toHaveBeenCalledTimes(1);
+    });
+
+    it("should set shouldIgnoreResize to true when window is being resized", () => {
+        const collapseHandler = new MockCollapseHandler() as unknown as CollapseHandler;
+        const behaviour = new CollapseOnWindowResizeBehaviour(collapseHandler);
+        expect(behaviour.shouldIgnoreResize).toBe(false);
+        // When the window is being resized, this behaviour should tell the AutoCollapser to
+        // ignore any panel resize events.
+        UIStore.instance.isWindowBeingResized = true;
+        expect(behaviour.shouldIgnoreResize).toBe(true);
+    });
+
+    it("should not auto-collapse panel when user has manually resized the panel", () => {
+        const collapseHandler = new MockCollapseHandler();
+        const behaviour = new CollapseOnWindowResizeBehaviour(collapseHandler as unknown as CollapseHandler);
+        // Let's make the window smaller so that the panel is auto-collapsed.
+        UIStore.instance.emit(UI_EVENTS.WidthDecreased, 750);
+        expect(collapseHandler.collapse).toHaveBeenCalledTimes(1);
+        collapseHandler.collapse.mockClear();
+        // Let's say that the user now manually expands the auto-collapsed panel.
+        behaviour.onLeftPanelResized();
+        // Let's say that the window now became even smaller
+        UIStore.instance.emit(UI_EVENTS.WidthDecreased, 500);
+        // The panel should not be auto-collapsed again
+        expect(collapseHandler.collapse).not.toHaveBeenCalled();
+    });
+
+    it("should return correct shouldStartCollapsed", () => {
+        const collapseHandler = new MockCollapseHandler();
+        new CollapseOnWindowResizeBehaviour(collapseHandler as unknown as CollapseHandler);
+        // When the window is smaller than 768px, start collapsed.
+        UIStore.instance.windowWidth = 750;
+        expect(CollapseOnWindowResizeBehaviour.shouldStartCollapsed()).toBe(true);
+        // When the window is larger than 768px, start expanded.
+        UIStore.instance.windowWidth = 900;
+        expect(CollapseOnWindowResizeBehaviour.shouldStartCollapsed()).toBe(false);
+    });
+});
diff --git a/apps/web/test/viewmodels/structures/auto-collapse/mocks.ts b/apps/web/test/viewmodels/structures/auto-collapse/mocks.ts
new file mode 100644
index 0000000000..f3131a0c43
--- /dev/null
+++ b/apps/web/test/viewmodels/structures/auto-collapse/mocks.ts
@@ -0,0 +1,34 @@
+/*
+ * Copyright 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+export class MockCollapseHandler {
+    public collapse = jest.fn().mockImplementation(() => {
+        this.isAutoCollapsed = true;
+    });
+    public expand = jest.fn().mockImplementation(() => {
+        this.isAutoCollapsed = false;
+    });
+    public isAutoCollapsed = false;
+    public panelHandle = new MockPanelHandle();
+}
+
+export class MockPanelHandle {
+    private _isCollapsed = false;
+    public inPixels = 100;
+    public isCollapsed = jest.fn().mockImplementation(() => {
+        return this._isCollapsed;
+    });
+    public collapse = jest.fn().mockImplementation(() => {
+        this._isCollapsed = true;
+    });
+    public resize = jest.fn().mockImplementation(() => {
+        this._isCollapsed = false;
+    });
+    getSize = jest.fn().mockReturnValue({
+        inPixels: this.inPixels,
+    });
+}