fix(web): started threads no longer offer a machine picker that does nothing

apps/web/src/components/BranchToolbar.panel.browser.test.tsx

@@ -106,6 +106,22 @@ it("keeps machine choices usable when the combined row's workspace is locked", a
   expect(onEnvironmentChange).toHaveBeenCalledWith("remote");
 });
 
+it("shows a started thread's primary machine as a static label in the composer", async () => {
+  await render({
+    layout: "composer",
+    showGitControls: false,
+    availableEnvironments: ["local", "remote"].map((id) => ({
+      environmentId: EnvironmentId.make(id),
+      projectId: ProjectId.make("project"),
+      label: id,
+      isPrimary: id === "local",
+      machine: "server",
+    })),
+  });
+  expect(container.textContent).toBe("local");
+  expect(container.querySelector("button")).toBeNull();
+});
+
 it.each([
   ["local", null, "/tmp/project"],
   ["worktree", null, null],

apps/web/src/components/BranchToolbar.tsx

@@ -645,14 +645,15 @@ export const BranchToolbar = memo(function BranchToolbar({
     ],
   );
 
-  const showEnvironmentPicker = Boolean(
-    availableEnvironments && availableEnvironments.length > 1 && onEnvironmentChange,
-  );
+  const hasMultipleEnvironments = (availableEnvironments?.length ?? 0) > 1;
+  const showEnvironmentPicker = hasMultipleEnvironments && onEnvironmentChange !== undefined;
   const activeEnvironmentOption =
     availableEnvironments?.find((env) => env.environmentId === environmentId) ?? null;
+  // Started threads get no handler but still show which machine they run on,
+  // matching the strip visibility ChatView computes from the same inputs.
   const showEnvironmentIndicator = shouldShowEnvironmentIndicator({
     activeEnvironment: activeEnvironmentOption,
-    canPickEnvironment: showEnvironmentPicker,
+    canPickEnvironment: hasMultipleEnvironments,
   });
   const [stripElement, setStripElement] = useState<HTMLDivElement | null>(null);
   const labelsOverflow = useLabelsOverflow(stripElement);
@@ -755,7 +756,7 @@ export const BranchToolbar = memo(function BranchToolbar({
                 envLocked={envLocked}
                 environmentId={environmentId}
                 availableEnvironments={availableEnvironments}
-                {...(showEnvironmentPicker && onEnvironmentChange ? { onEnvironmentChange } : {})}
+                {...(showEnvironmentPicker ? { onEnvironmentChange } : {})}
               />
               {showGitControls ? (
                 <Separator

apps/web/src/components/ChatView.tsx

@@ -2178,6 +2178,11 @@ export default function ChatView(props: ChatViewProps) {
       ? null
       : clampFileAttachmentUploadBytes(advertisedFileAttachmentBytes);
   const envLocked = Boolean(activeThread && (activeMessageCount > 0 || activeRuntime !== null));
+  // onEnvironmentChange only retargets an unsent draft. envLocked alone does not
+  // rule out a started thread: one whose machine is offline never loads its
+  // messages, so it would otherwise be offered a machine picker that does nothing.
+  const canChangeEnvironment =
+    hasMultipleEnvironments && !envLocked && draftId !== null && !draftThread?.promotedTo;
 
   const loadBalancingSettings = useClientSettings((settings) => ({
     loadBalancingEnabled: settings.loadBalancingEnabled,
@@ -5607,7 +5612,7 @@ export default function ChatView(props: ChatViewProps) {
       "composer.mode": openComposerControl("composer.mode"),
       "composer.workspace": openComposerControl("composer.workspace"),
       "composer.cycleHost": ({ event }) => {
-        if (envLocked || !draftId || !hasMultipleEnvironments) return false;
+        if (!canChangeEnvironment) return false;
         if (event.repeat) return;
         const currentId = readPendingEnvironmentChange()?.environmentId ?? environmentId;
         const index = logicalProjectEnvironments.findIndex(
@@ -8373,7 +8378,7 @@ export default function ChatView(props: ChatViewProps) {
                                 {...(canCheckoutPullRequestIntoThread
                                   ? { onCheckoutPullRequestRequest: openPullRequestDialogStable }
                                   : {})}
-                                {...(hasMultipleEnvironments
+                                {...(canChangeEnvironment
                                   ? { onEnvironmentChange: onEnvironmentChangeStable }
                                   : {})}
                                 autoEnvironmentLabel={autoEnvironmentLabel}
@@ -8453,7 +8458,7 @@ export default function ChatView(props: ChatViewProps) {
                   ? onAutoEnvironmentStable
                   : undefined
               }
-              onEnvironmentChange={onEnvironmentChangeStable}
+              {...(canChangeEnvironment ? { onEnvironmentChange: onEnvironmentChangeStable } : {})}
               onEnvModeChange={onEnvModeChangeStable}
               envMode={envMode}
               {...(canOverrideServerThreadEnvMode

apps/web/src/components/chat/ThreadDetailsPanel.tsx

@@ -44,7 +44,7 @@ export interface ThreadDetailsPanelProps extends Pick<
   availableEnvironments: readonly EnvironmentOption[];
   autoEnvironmentLabel?: string | undefined;
   onAutoEnvironment?: (() => void) | undefined;
-  onEnvironmentChange: (environmentId: EnvironmentId) => void;
+  onEnvironmentChange?: (environmentId: EnvironmentId) => void;
   onEnvModeChange: (mode: EnvMode) => void;
   envMode: EnvMode;
   activeThreadBranchOverride?: string | null;
@@ -113,7 +113,9 @@ export function ThreadDetailsPanel(props: ThreadDetailsPanelProps) {
                   layout="panel"
                   panelSection="workspace"
                   availableEnvironments={props.availableEnvironments}
-                  onEnvironmentChange={props.onEnvironmentChange}
+                  {...(props.onEnvironmentChange
+                    ? { onEnvironmentChange: props.onEnvironmentChange }
+                    : {})}
                   autoEnvironmentLabel={props.autoEnvironmentLabel}
                   onAutoEnvironment={props.onAutoEnvironment}
                   {...branchToolbarProps}