Refresh open file tabs when revisited (#445) (#1699)

File previews read once and then froze: visited tabs stay mounted-but-hidden
(LRU in useMountedTabSet), so refetchOnMount never re-fired and the only way
to see fresh content was to close and reopen the tab.

Gate the file query's `enabled` on tab + app visibility so React Query
refetches on the disabled->enabled transition — and only when the cached read
is stale (keeping the existing 5s staleTime), so a quick flip away and back
doesn't re-read. Covers switching between tabs and backgrounding/reopening the
whole app to the same tab.

Adds a wiring test covering the stale-revisit refetch, the fresh-revisit
debounce, the app-foreground refetch, and the read-error path.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

packages/app/src/components/file-pane-enabled.test.ts

@@ -0,0 +1,28 @@
+import { describe, expect, it } from "vitest";
+import { isFileQueryEnabled } from "./file-pane-enabled";
+
+describe("isFileQueryEnabled", () => {
+  it("reads when there is a target, the tab is active, and the app is visible", () => {
+    expect(isFileQueryEnabled({ hasReadTarget: true, isTabActive: true, isAppVisible: true })).toBe(
+      true,
+    );
+  });
+
+  it("does not read while the tab is hidden", () => {
+    expect(
+      isFileQueryEnabled({ hasReadTarget: true, isTabActive: false, isAppVisible: true }),
+    ).toBe(false);
+  });
+
+  it("does not read while the app is backgrounded", () => {
+    expect(
+      isFileQueryEnabled({ hasReadTarget: true, isTabActive: true, isAppVisible: false }),
+    ).toBe(false);
+  });
+
+  it("does not read without a resolved file target", () => {
+    expect(
+      isFileQueryEnabled({ hasReadTarget: false, isTabActive: true, isAppVisible: true }),
+    ).toBe(false);
+  });
+});

packages/app/src/components/file-pane-enabled.ts

@@ -0,0 +1,17 @@
+/**
+ * Whether `FilePane` should read its file right now.
+ *
+ * The read is gated on visibility so a revisited tab refetches instead of showing
+ * the frozen first-load snapshot (#445): React Query refetches on the
+ * disabled→enabled transition (stale-gated by the query's staleTime). The file is
+ * read only when there is something to read AND the pane can actually show it —
+ * the tab is the active one (not a hidden, mounted-but-offscreen tab) and the
+ * whole app is in the foreground.
+ */
+export function isFileQueryEnabled(input: {
+  hasReadTarget: boolean;
+  isTabActive: boolean;
+  isAppVisible: boolean;
+}): boolean {
+  return input.hasReadTarget && input.isTabActive && input.isAppVisible;
+}

packages/app/src/components/file-pane.tsx

@@ -1,4 +1,4 @@
-import React, { useEffect, useMemo, useRef } from "react";
+import React, { useContext, useEffect, useMemo, useRef } from "react";
 import { useQuery } from "@tanstack/react-query";
 import type { FileReadResult } from "@getpaseo/client/internal/daemon-client";
 import {
@@ -29,6 +29,9 @@ import { createPreviewAttachmentId, getFileNameFromPath } from "@/attachments/ut
 import { explorerFileFromReadResult } from "@/file-explorer/read-result";
 import { resolveFilePreviewReadTarget } from "@/file-explorer/preview-target";
 import type { WorkspaceFileLocation } from "@/workspace/file-open";
+import { MountedTabActiveContext } from "@/components/split-container";
+import { useAppVisible } from "@/hooks/use-app-visible";
+import { isFileQueryEnabled } from "@/components/file-pane-enabled";
 
 interface CodeLineProps {
   tokens: HighlightToken[];
@@ -408,9 +411,19 @@ export function FilePane({
     [normalizedFilePath, normalizedWorkspaceRoot],
   );
 
+  // Re-read the file when this pane becomes visible again (#445). `isActive`
+  // covers tab switches, `isAppVisible` the whole-app background/foreground; the
+  // gate itself lives in isFileQueryEnabled.
+  const isActive = useContext(MountedTabActiveContext);
+  const isAppVisible = useAppVisible();
+
   const query = useQuery({
     queryKey: ["workspaceFile", serverId, readTarget?.cwd ?? null, readTarget?.path ?? null],
-    enabled: Boolean(client && readTarget),
+    enabled: isFileQueryEnabled({
+      hasReadTarget: Boolean(client && readTarget),
+      isTabActive: isActive,
+      isAppVisible,
+    }),
     queryFn: async () => {
       if (!client || !readTarget) {
         return {