fix(docker): pre-fill the copied case instead of blanking two fields

The previous version cleared the case name and the in-container directory on the
grounds that they must differ. That left a form with three fields mysteriously
filled and two empty, and turned the most common operation — changing
/srv/app/api to /srv/app/web — into retyping a long path.

Both are now pre-filled, with focus on the in-container directory and the caret
at the end, since the tail is what changes. What stops an unmodified submit is no
longer an empty field but a guard: the values applied are recorded, compared at
submit time, and if nothing changed the reason is stated next to the field and
focus moves to it, without sending a request that is certain to be refused.

The server refuses these anyway (a duplicate case name, a twin case on the same
container and directory) and its errors are clear; but making a round trip to be
told "you forgot to edit the field you are looking at" is worse than saying so on
the spot. The guard only applies when a source case was actually selected, so
filling the adopt form from scratch is unaffected.

⚠️ The status text is written into dockerLinkStatus. My first version referenced
an id that does not exist (dockerAdoptStatus), which made the explanation vanish
silently and left only a toast. The test now extracts that id from the code and
looks it up in index.html, pinning that it must really exist.

src/web/public/session-ui.js

@@ -3196,6 +3196,7 @@ Object.assign(CodemanApp.prototype, {
       option.dataset.container = c.docker.container;
       option.dataset.hostId = c.docker.hostId;
       option.dataset.path = c.docker.path;
+      option.dataset.workdir = c.docker.containerWorkdir || c.docker.path;
       select.appendChild(option);
     }
     // Nothing to duplicate yet: an empty picker is noise on the first adoption.
@@ -3222,9 +3223,49 @@ Object.assign(CodemanApp.prototype, {
     set('dockerContainerName', option.dataset.container);
     set('dockerHostId', option.dataset.hostId);
     set('dockerWorkspacePath', option.dataset.path);
-    set('dockerCaseName', '');
-    set('dockerAdoptWorkdir', '');
-    document.getElementById('dockerAdoptWorkdir')?.focus();
+    // Pre-filled, NOT cleared: these two must differ from the source, but editing
+    // `/srv/app/api` into `/srv/app/web` beats retyping a long path, and the same
+    // goes for the name. What keeps a duplicate from being submitted unchanged is
+    // the guard below (dockerCloneGuard), which is a better trade than an empty
+    // field: the form stays a starting point instead of a blank form with three
+    // fields mysteriously filled in.
+    set('dockerCaseName', option.value);
+    set('dockerAdoptWorkdir', option.dataset.workdir);
+    // Remembered so the guard can tell "unchanged" from "happens to look similar".
+    select.dataset.appliedName = option.value;
+    select.dataset.appliedWorkdir = option.dataset.workdir || '';
+    const workdir = document.getElementById('dockerAdoptWorkdir');
+    workdir?.focus();
+    // Caret at the end: the tail is the part that changes.
+    if (workdir) workdir.setSelectionRange(workdir.value.length, workdir.value.length);
+  },
+
+  /**
+   * Refuse a duplicate that still carries the source case's name or directory.
+   *
+   * Both are pre-filled so they can be EDITED, which means both can also be left
+   * alone by accident. The server refuses either (an existing case name, or an
+   * exact same-container-same-directory twin) with a clear message, but a
+   * round-trip to be told "you forgot to change the field you were looking at" is
+   * worse than saying so here, next to the field, before anything is sent.
+   *
+   * Returns the offending element, or null when the form is fine.
+   */
+  dockerCloneGuard() {
+    const select = document.getElementById('dockerAdoptCloneFrom');
+    if (!select || !select.value) return null;
+    const name = document.getElementById('dockerCaseName');
+    const workdir = document.getElementById('dockerAdoptWorkdir');
+    if (name && name.value.trim() === (select.dataset.appliedName || '')) {
+      return { el: name, message: `"${name.value.trim()}" is the case you copied from — give this one a new name.` };
+    }
+    if (workdir && workdir.value.trim() === (select.dataset.appliedWorkdir || '')) {
+      return {
+        el: workdir,
+        message: 'Same container and same directory as the case you copied from — point this one at another directory.',
+      };
+    }
+    return null;
   },
 
   async _loadDockerContainerOptions() {
@@ -3327,6 +3368,17 @@ Object.assign(CodemanApp.prototype, {
       this.showToast('Enter the name of the running container to attach to', 'error');
       return;
     }
+    // A duplicate that still carries the source's name or directory: say so here,
+    // beside the field, rather than sending a request certain to come back refused.
+    const cloneIssue = adopting ? this.dockerCloneGuard() : null;
+    if (cloneIssue) {
+      this.showToast(cloneIssue.message, 'error');
+      const statusEl = document.getElementById('dockerLinkStatus');
+      if (statusEl) statusEl.textContent = cloneIssue.message;
+      cloneIssue.el.focus();
+      cloneIssue.el.select?.();
+      return;
+    }
 
     try {
       if (statusEl) {

test/docker-adopt-duplicate.test.ts

@@ -55,8 +55,8 @@ describe('the picker only offers what the server would accept', () => {
   });
 });
 
-describe('applying a source fills what stays and clears what must differ', () => {
-  const fn = ui.slice(ui.indexOf('applyDockerCloneSource()'), ui.indexOf('applyDockerCloneSource()') + 1400);
+describe('applying a source fills every field, including the two that must differ', () => {
+  const fn = ui.slice(ui.indexOf('applyDockerCloneSource()'), ui.indexOf('dockerCloneGuard()'));
 
   it('carries over container, host and workspace', () => {
     for (const id of ['dockerContainerName', 'dockerHostId', 'dockerWorkspacePath']) {
@@ -64,15 +64,21 @@ describe('applying a source fills what stays and clears what must differ', () =>
     }
   });
 
-  it('clears the case name and the container workdir', () => {
-    // Keeping either would pre-fill a value the server is certain to refuse —
-    // the name as an existing case, the workdir as an exact twin.
-    expect(fn).toContain("set('dockerCaseName', '')");
-    expect(fn).toContain("set('dockerAdoptWorkdir', '')");
+  it('PRE-FILLS the case name and container workdir rather than clearing them', () => {
+    // Editing `/srv/app/api` into `/srv/app/web` beats retyping a long path, and a
+    // form with three fields mysteriously filled and two blank reads as broken.
+    // What stops an unchanged submit is the guard, not an empty field.
+    expect(fn).toContain("set('dockerCaseName', option.value)");
+    expect(fn).toContain("set('dockerAdoptWorkdir', option.dataset.workdir)");
   });
 
-  it('focuses the workdir, the field the user came here to change', () => {
-    expect(fn).toMatch(/getElementById\('dockerAdoptWorkdir'\)\?\.focus\(\)/);
+  it('remembers what it applied, so the guard can tell unchanged from similar', () => {
+    expect(fn).toContain('select.dataset.appliedName = option.value');
+    expect(fn).toContain('select.dataset.appliedWorkdir =');
+  });
+
+  it('focuses the workdir with the caret at the END, where the edit happens', () => {
+    expect(fn).toMatch(/setSelectionRange\(workdir\.value\.length, workdir\.value\.length\)/);
   });
 
   it('does nothing for the blank "start from scratch" option', () => {
@@ -80,6 +86,39 @@ describe('applying a source fills what stays and clears what must differ', () =>
   });
 });
 
+describe('the guard refuses a duplicate that was never edited', () => {
+  const fn = ui.slice(ui.indexOf('dockerCloneGuard()'), ui.indexOf('dockerCloneGuard()') + 1400);
+
+  it('flags an unchanged case name', () => {
+    expect(fn).toMatch(/appliedName/);
+    expect(fn).toContain('give this one a new name');
+  });
+
+  it('flags an unchanged container workdir', () => {
+    expect(fn).toMatch(/appliedWorkdir/);
+    expect(fn).toContain('another directory');
+  });
+
+  it('stays silent when no source was picked', () => {
+    // Typing a fresh adoption by hand must not be second-guessed.
+    expect(fn).toMatch(/if \(!select \|\| !select\.value\) return null;/);
+  });
+
+  it('runs BEFORE the request, and focuses the offending field', () => {
+    const submit = ui.slice(ui.indexOf('const cloneIssue'), ui.indexOf('const cloneIssue') + 500);
+    expect(submit).toContain('cloneIssue.el.focus()');
+    expect(submit).toContain('return;');
+  });
+
+  it('reports into a status element that actually exists', () => {
+    // A dead id would silently drop the explanation next to the field.
+    const submit = ui.slice(ui.indexOf('const cloneIssue'), ui.indexOf('const cloneIssue') + 500);
+    const id = /getElementById\('([^']+)'\)/.exec(submit)?.[1];
+    expect(id).toBeTruthy();
+    expect(html).toContain(`id="${id}"`);
+  });
+});
+
 describe('the row is wired into the adoption panel', () => {
   it('lives in the adopt-only block and starts hidden', () => {
     expect(html).toMatch(/id="dockerAdoptCloneRow"[^>]*hidden/);