fix(ui): keep the workspace in the address on a project switch (A5)

The project switcher turned "?project=django-demo&workspace=galaxy" into
"?project=cbm", so a reload could open another workspace. The new
address now keeps the page-wide parameters App names (the workspace and
the walk bounds) and nothing of the old project, and the address always
names the workspace in view, also one carried over a switch or chosen
from storage. Browser Back and Forward between projects work as before.

Signed-off-by: Bernhard Jackiewicz <bj@techport.io>

graph-ui/src/App.tsx

@@ -311,7 +311,7 @@ import type { SettingsMeasurement } from './settings/SettingsPanel';
 import AddProjectIndexDialog from './projects/AddProjectIndexDialog';
 import ProjectSwitcher from './projects/ProjectSwitcher';
 import type { ProjectsSource } from './projects/ProjectsPanel';
-import { ProjectWindows, type ProjectWindow } from './app/project-windows';
+import { ProjectWindows, useParamInAddress, type ProjectWindow } from './app/project-windows';
 import { MODEL_SUGGESTIONS, fetchCommand } from './settings/model-catalog';
 import { modelKey, readModelPreference, recordModelPreference } from './settings/model-preference';
 import {
@@ -758,10 +758,12 @@ interface WindowCarry {
 /**
  * Jedes Projekt bekommt ein frisches Fenster in derselben Seite (K24, Kopf von
  * src/app/project-windows.tsx). Der Agent reicht beim Wechsel sein geladenes
- * Modell an das Fenster des neuen Projekts weiter.
+ * Modell an das Fenster des neuen Projekts weiter. Die Adresse behaelt dabei
+ * den Arbeitsbereich und die Grenzen des Walks, nichts vom alten Projekt (A5).
  */
+const PAGE_PARAMS = ['workspace', CLOSURE_DEPTH_PARAM, CLOSURE_CAP_PARAM] as const;
 export default function App(): JSX.Element {
-    return <ProjectWindows<WindowCarry> around={switchKeepingAgent}>
+    return <ProjectWindows<WindowCarry> around={switchKeepingAgent} keepParams={PAGE_PARAMS}>
         {shown => <ProjectWindowView key={shown.key} projectWindow={shown} />}
     </ProjectWindows>;
 }
@@ -783,11 +785,10 @@ function ProjectWindowView({ projectWindow }: { projectWindow: ProjectWindow<Win
             setProactiveSelection(undefined); setChatGraphSelection(undefined); setChatAttachment(undefined);
         }
         setWorkspace(value);
-        const url = new URL(window.location.href);
-        url.searchParams.set('workspace', value);
-        window.history.replaceState(window.history.state, '', url);
         try { localStorage.setItem('cbm.workspace', value); } catch { /* Session-only preference. */ }
     };
+    // The address always names the workspace in view, also one carried over a project switch, so a reload keeps it (A5).
+    useParamInAddress('workspace', workspace);
     const [welcomeOpen, setWelcomeOpen] = useState(() => {
         try { return localStorage.getItem('cbm.workspace.setup') !== 'done'; } catch { return false; }
     });

graph-ui/src/app/project-windows.test.tsx

@@ -2,7 +2,7 @@
 import { act, useEffect } from 'react';
 import { createRoot, type Root } from 'react-dom/client';
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
-import { ProjectWindows, type ProjectWindow } from './project-windows';
+import { ProjectWindows, useParamInAddress, type ProjectWindow } from './project-windows';
 
 /*
  * K24: a project switch stays in the page. Each project gets a fresh window (fresh state in
@@ -77,6 +77,34 @@ describe('an in-page project switch (K24)', () => {
         expect(mounted).toHaveLength(3);
     });
 
+    /* Hand test 2026-10-04 (A5): the switcher turned "?project=django-demo&workspace=galaxy" into "?project=cbm". */
+    it('keeps the page-wide parameters it is given, the workspace among them, and nothing project specific', async () => {
+        window.history.replaceState(null, '', '/?project=django-demo&workspace=galaxy&codeatlasClosureDepth=4&file=django%2Fshortcuts.py');
+        await act(async () => root.render(<ProjectWindows<Carry> keepParams={['workspace', 'codeatlasClosureDepth', 'codeatlasClosureCap']}>
+            {shown => <Window key={shown.key} window={shown} />}</ProjectWindows>));
+        await act(async () => current!.open('atlas sample'));
+        expect(window.location.search).toBe('?project=atlas%20sample&workspace=galaxy&codeatlasClosureDepth=4');
+        expect(shownProject()).toBe('atlas sample');
+        // Back and Forward of the browser still switch between both projects, each with its own address.
+        const popped = () => new Promise<void>(resolve => window.addEventListener('popstate', () => resolve(), { once: true }));
+        await act(async () => { const done = popped(); window.history.back(); await done; });
+        expect(window.location.search).toBe('?project=django-demo&workspace=galaxy&codeatlasClosureDepth=4&file=django%2Fshortcuts.py');
+        expect(shownProject()).toBe('django-demo');
+        await act(async () => { const done = popped(); window.history.forward(); await done; });
+        expect(shownProject()).toBe('atlas sample');
+    });
+
+    it('writes the workspace in view into the address without a new history entry', async () => {
+        window.history.replaceState(null, '', '/?project=django-demo');
+        const length = window.history.length;
+        function Workspace({ name }: { name: string }) { useParamInAddress('workspace', name); return null; }
+        await act(async () => root.render(<Workspace name="architecture" />));
+        expect(window.location.search).toBe('?project=django-demo&workspace=architecture');
+        await act(async () => root.render(<Workspace name="galaxy" />));
+        expect(window.location.search).toBe('?project=django-demo&workspace=galaxy');
+        expect(window.history.length).toBe(length);
+    });
+
     it('commits the switch inside the frame it is given, so the old window is gone and the new one there when it ends', async () => {
         const seen: string[] = [];
         const around = (commit: () => void) => { seen.push(`before:${mounted.join(',')}`); commit(); seen.push(`after:${mounted.join(',')}|${unmounted.join(',')}`); };

graph-ui/src/app/project-windows.tsx

@@ -33,6 +33,30 @@ export function projectOfLocation(search = window.location.search): string {
     return new URLSearchParams(search).get('project') ?? '';
 }
 
+/*
+ * Handtest vom 2026-10-04 (A5): der Wechsel machte aus
+ * "?project=django-demo&workspace=galaxy" die Adresse "?project=cbm", und ein
+ * Neuladen danach konnte einen anderen Arbeitsbereich zeigen. Die Adresse des
+ * neuen Projekts behaelt nun die Parameter der ganzen Seite, die der Aufrufer
+ * nennt (den Arbeitsbereich, die Grenzen des Walks), und nichts, was zum alten
+ * Projekt gehoert.
+ */
+export function projectSwitchHref(project: string, search: string, keep: readonly string[]): string {
+    const from = new URLSearchParams(search);
+    const kept = keep.flatMap(name => { const value = from.get(name); return value === null ? [] : [`&${encodeURIComponent(name)}=${encodeURIComponent(value)}`]; });
+    return `${projectHref(project)}${kept.join('')}`;
+}
+
+/** Haelt einen Parameter der Adresse auf dem Wert, den die Seite zeigt, ohne neuen Eintrag im Verlauf (A5: der Arbeitsbereich). */
+export function useParamInAddress(name: string, value: string): void {
+    useEffect(() => {
+        const url = new URL(window.location.href);
+        if (url.searchParams.get(name) === value) return;
+        url.searchParams.set(name, value);
+        window.history.replaceState(window.history.state, '', url);
+    }, [name, value]);
+}
+
 export interface ProjectWindow<C> {
     /** Wechselt mit jedem Projekt; als React-Schluessel des Fensters gedacht. */
     key: number;
@@ -50,9 +74,11 @@ export interface ProjectWindow<C> {
 
 interface Shown<C> { key: number; project: string; controller: AbortController; carried?: C }
 
-export function ProjectWindows<C>({ children, around = commit => commit() }: {
+export function ProjectWindows<C>({ children, around = commit => commit(), keepParams = [] }: {
     children: (window: ProjectWindow<C>) => ReactNode;
     around?: (commit: () => void) => void;
+    /** Parameter der ganzen Seite, die ein Wechsel in die neue Adresse mitnimmt (A5). */
+    keepParams?: readonly string[];
 }): JSX.Element {
     const [shown, setShown] = useState<Shown<C>>(() => ({ key: 0, project: projectOfLocation(), controller: new AbortController() }));
     const current = useRef(shown); current.current = shown;
@@ -65,9 +91,10 @@ export function ProjectWindows<C>({ children, around = commit => commit() }: {
         around(() => flushSync(() => setShown(next)));
         previous.controller.abort();
     }, [around]);
+    const keep = useRef(keepParams); keep.current = keepParams;
     const open = useCallback((project: string) => {
         if (project === current.current.project) return;
-        window.history.pushState(null, '', projectHref(project));
+        window.history.pushState(null, '', projectSwitchHref(project, window.location.search, keep.current));
         show(project);
     }, [show]);
     const report = useCallback((value: C) => { carry.current = value; }, []);