diff --git a/website/src/lib/components/map/layer-control/extension-api.test.ts b/website/src/lib/components/map/layer-control/extension-api.test.ts
index 3fee37769..8fab8157a 100644
--- a/website/src/lib/components/map/layer-control/extension-api.test.ts
+++ b/website/src/lib/components/map/layer-control/extension-api.test.ts
@@ -399,13 +399,43 @@ describe('ExtensionAPI v2', () => {
         assert.deepEqual(calls, [{ a: true }]);
     });
 
-    it('does not tell the extension that all is hidden when the page closes', () => {
-        const calls: unknown[] = [];
-        api.onVisibilityChange('Heatmaps', (visible) => calls.push(visible));
-        api.setOverlays('Heatmaps', [v2('a', { visible: true })]);
-        (api as unknown as { destroy(): void }).destroy();
-        assert.deepEqual(calls, []);
+    it('forgets the state of the overlays with the next session', () => {
+        const saved = () => ({
+            currentOverlays: JSON.stringify(get(settings.currentOverlays)),
+            previousOverlays: JSON.stringify(get(settings.previousOverlays)),
+            selectedOverlayTree: JSON.stringify(get(settings.selectedOverlayTree)),
+            opacities: JSON.stringify(get(settings.opacities)),
+        });
+        // what was saved, not the default trees, which are the ones that are allowed
+        settings.connect(saved());
+
+        api.setOverlays('Heatmaps', [v2('a', { visible: true }), v2('b')]);
+        api.addOrUpdateOverlay(overlay('one'));
+        settings.selectedOverlayTree.update((tree) => {
+            group(tree)![id('b')] = false;
+            return tree;
+        });
+        settings.opacities.update((opacities) => ({ ...opacities, [id('a')]: 0.3 }));
+        const stored = saved();
+        assert.ok(stored.currentOverlays.includes(id('a')));
+
+        settings.connect(stored);
+        for (const tree of [
+            get(settings.currentOverlays),
+            get(settings.previousOverlays),
+            get(settings.selectedOverlayTree),
+        ]) {
+            assert.ok(!JSON.stringify(tree).includes('extension-'));
+        }
+        assert.equal(get(settings.opacities)[id('a')], 0.3);
+
+        // an extension that is still there starts again, as the source of what it remembers
+        const next = new ExtensionAPI();
+        next.setOverlays('Heatmaps', [v2('a'), v2('b', { visible: true })]);
         assert.equal(isSelected(get(settings.currentOverlays)!, id('a')), false);
+        assert.equal(isSelected(get(settings.currentOverlays)!, id('b')), true);
+        assert.equal(isSelected(get(settings.selectedOverlayTree), id('b')), true);
+        next.setOverlays('Heatmaps', []);
     });
 
     it('leaves a style URL for the map to fetch', () => {
diff --git a/website/src/lib/components/map/layer-control/extension-api.ts b/website/src/lib/components/map/layer-control/extension-api.ts
index ac08bf550..2e2a90373 100644
--- a/website/src/lib/components/map/layer-control/extension-api.ts
+++ b/website/src/lib/components/map/layer-control/extension-api.ts
@@ -1,7 +1,7 @@
 import { settings } from '$lib/state/settings';
 import type { StyleSpecification } from 'maplibre-gl';
 import { derived, get, writable, type Writable } from 'svelte/store';
-import { isSelected, prefixStyle, remove, removeAll } from './utils';
+import { isSelected, prefixStyle, removeAll } from './utils';
 import { layerGroup, overlays, overlayTree, type LayerTreeType } from '$lib/data/layers';
 import { browser } from '$app/environment';
 import { map } from '$lib/components/map/map';
@@ -101,9 +101,6 @@ export class ExtensionAPI {
             Object.defineProperty(window, 'gpxstudio', {
                 value: this,
             });
-            addEventListener('beforeunload', () => {
-                this.destroy();
-            });
         }
     }
 
@@ -432,30 +429,6 @@ export class ExtensionAPI {
         const names = new Set(Array.from($overlays.values(), (overlay) => overlay.extensionName));
         return (id: string) => names.has(id);
     });
-
-    private destroy() {
-        // the page is closing: the user has not hidden anything, which extensions would then save
-        this._visibilityCallbacks.clear();
-        const ids = Array.from(get(this._overlays).keys());
-        currentOverlays.update((current) => {
-            ids.forEach((id) => {
-                remove(current, id);
-            });
-            return current;
-        });
-        previousOverlays.update((previous) => {
-            ids.forEach((id) => {
-                remove(previous, id);
-            });
-            return previous;
-        });
-        selectedOverlayTree.update((selected) => {
-            ids.forEach((id) => {
-                remove(selected, id);
-            });
-            return selected;
-        });
-    }
 }
 
 export const extensionAPI = new ExtensionAPI();
diff --git a/website/src/lib/state/setting.test.ts b/website/src/lib/state/setting.test.ts
index 3a5e6686a..5422fc41e 100644
--- a/website/src/lib/state/setting.test.ts
+++ b/website/src/lib/state/setting.test.ts
@@ -141,11 +141,13 @@ describe('layer trees', () => {
         assert.deepEqual(filterLayerTree({}, allowed), allowed);
     });
 
-    it('drops the layers that are not allowed anymore, but not the custom ones', () => {
+    it('drops the layers that are not allowed anymore and those of extensions, not the custom ones', () => {
         const filtered = filterLayerTree(
             {
-                world: { a: true, gone: true, 'custom-1': true, 'extension-x': false },
+                world: { a: true, gone: true, 'custom-1': true, 'extension-x': true },
                 old: { x: true },
+                extension: { 'extension-Name/y': true },
+                custom: { 'custom-2': false },
             },
             allowed
         );
@@ -153,10 +155,10 @@ describe('layer trees', () => {
             a: true,
             b: false,
             'custom-1': true,
-            'extension-x': false,
         });
-        assert.equal('old' in filtered, true);
-        assert.deepEqual(filtered.old, {});
+        assert.equal('old' in filtered, false);
+        assert.equal('extension' in filtered, false);
+        assert.deepEqual(filtered.custom, { 'custom-2': false });
     });
 
     it('does not take a branch for a layer, or a layer for a branch', () => {
diff --git a/website/src/lib/state/setting.ts b/website/src/lib/state/setting.ts
index ca6234500..6189e4af7 100644
--- a/website/src/lib/state/setting.ts
+++ b/website/src/lib/state/setting.ts
@@ -196,7 +196,9 @@ function isRecord(value: unknown): value is Record<string, unknown> {
 
 /**
  * The saved tree `t`, with the layers of `allowed`: the choices saved for them are kept, the new
- * ones have their default, and of the others only the custom and extension layers are kept.
+ * ones have their default, and of the others only the custom layers are kept. The layers of
+ * extensions are saved with the others, but not read back: they come again when an extension sets
+ * them, if it is still there.
  */
 export function filterLayerTree(t: unknown, allowed: LayerTreeType | undefined): LayerTreeType {
     const tree = isRecord(t) ? t : {};
@@ -214,11 +216,14 @@ export function filterLayerTree(t: unknown, allowed: LayerTreeType | undefined):
     Object.entries(tree).forEach(([key, value]) => {
         if (!Object.hasOwn(filtered, key)) {
             if (typeof value === 'boolean') {
-                if (key.startsWith('custom-') || key.startsWith('extension-')) {
+                if (key.startsWith('custom-')) {
                     filtered[key] = value;
                 }
             } else if (isRecord(value)) {
-                filtered[key] = filterLayerTree(value, undefined);
+                const group = filterLayerTree(value, undefined);
+                if (Object.keys(group).length > 0) {
+                    filtered[key] = group;
+                }
             }
         }
     });