fix(runtime): don't hide the cursor for an empty interactive app (Ink parity) (#140)

vue-tui hid the terminal cursor EAGERLY at mount regardless of content, so an
interactive app whose root renders nothing emitted `\x1b[?25l` and hid the
user's cursor. Ink hides LAZILY (log-update, on the first render that writes)
and skips log-update entirely for an empty frame, so an empty app emits zero
cursor escapes.

Remove the eager mount-time hide and rely on log-update's lazy hide. That alone
was insufficient: an empty frame becomes "\n", and the old commit gate
`willRender(outputToRender) || isCursorDirty()` was true for "\n", so log-update
(and its lazy hide) was still reached. Align the outer commit gate to Ink's
exact condition (ink.tsx:1094) `output !== frameState.lastOutput || isCursorDirty()`,
comparing the RAW frame; on an empty first commit both are "" so log-update is
never reached. `willRender` is retained only for the inner BSU/ESU wrap gate.

Verified via PTY: empty app = 0 hides; non-empty = 1 lazy hide; useCursor =
hide-then-show within one render (SHOW last, cursor positioned). alt-screen,
screen-reader, and non-TTY cursor behavior unchanged.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/runtime-tests/integration/pty/cursor-empty-app.test.ts

@@ -0,0 +1,42 @@
+// Lazy cursor-hide parity (Ink v7.0.4). Ink hides the cursor LAZILY: log-update
+// hides on the first render that actually writes (log-update.ts:55-59), and the
+// onRender outer gate `output !== lastOutput || log.isCursorDirty()`
+// (ink.tsx:1094) skips log-update entirely for an empty frame (both ""). So an
+// interactive app whose root renders nothing emits ZERO cursor escapes; vue-tui
+// must match. Non-empty + useCursor apps still hide on the first render (the
+// lazy hide), so the cursor lifecycle is preserved.
+//
+// These run under a real PTY (run() spawns a TTY child with FORCE_COLOR=3 +
+// CI=false) so the genuine interactive log-update path is exercised, not the
+// debug helper.
+import { test as it, expect } from "vite-plus/test";
+import { run } from "./helpers/run.ts";
+
+const HIDE = "\x1b[?25l";
+const SHOW = "\x1b[?25h";
+
+it("interactive empty app (() => null) emits NO cursor-hide escape", async () => {
+  const output = await run("cursor-empty-app");
+  expect(output).toContain("exited");
+  // The bug: vue-tui eagerly hid the cursor at mount even though nothing
+  // renders. Ink emits zero cursor escapes for an empty frame.
+  expect(output).not.toContain(HIDE);
+});
+
+it("interactive non-empty app still hides the cursor on first render", async () => {
+  const output = await run("cursor-nonempty-app");
+  expect(output).toContain("exited");
+  // The lazy hide (log-update render) covers the non-empty case.
+  expect(output).toContain(HIDE);
+});
+
+it("useCursor app: last cursor visibility change is SHOW (cursor visible + positioned)", async () => {
+  const output = await run("cursor-usecursor-app");
+  expect(output).toContain("exited");
+  // log-update hides-then-shows within one render; the SHOW must come last so
+  // the cursor stays visible at the requested position.
+  expect(output).toContain(SHOW);
+  expect(output.lastIndexOf(SHOW)).toBeGreaterThan(output.lastIndexOf(HIDE));
+  // cursorTo(x=2) -> "\x1b[3G": the cursor is placed at the useCursor position.
+  expect(output).toContain("\x1b[3G");
+});

packages/runtime-tests/integration/pty/fixtures/cursor-empty-app.tsx

@@ -0,0 +1,25 @@
+import process from "node:process";
+import { createApp, useApp } from "@vue-tui/runtime";
+import { defineComponent, onMounted } from "vue";
+
+// An interactive app whose ROOT renders nothing. Ink emits ZERO cursor escapes
+// for an empty frame (its onRender outer gate `output !== lastOutput` is false
+// when both are "", so log-update — and its lazy hide — is never reached, and
+// the only mount-time hide lives in setAlternateScreen). vue-tui must match:
+// a no-content interactive app must NOT hide the terminal cursor.
+//
+// rawMode "auto" so the no-input app does not depend on the lifetime raw-mode
+// hold; we exit explicitly after signalling readiness so the PTY run resolves.
+const App = defineComponent(() => {
+  const { exit } = useApp();
+  onMounted(() => {
+    process.stdout.write("__READY__");
+    setTimeout(() => exit(), 100);
+  });
+  return () => null;
+});
+
+const app = createApp(App);
+app.mount({ rawMode: "auto", exitOnCtrlC: false });
+await app.waitUntilExit();
+console.log("exited");

packages/runtime-tests/integration/pty/fixtures/cursor-nonempty-app.tsx

@@ -0,0 +1,21 @@
+import process from "node:process";
+import { createApp, Text, useApp } from "@vue-tui/runtime";
+import { defineComponent, h, onMounted } from "vue";
+
+// A non-empty interactive app. The first frame has content, so log-update's
+// render() runs and its lazy hide fires — the cursor MUST be hidden on the first
+// render, matching Ink. This proves the lazy hide fully covers the non-empty
+// case once the eager mount-time hide is removed.
+const App = defineComponent(() => {
+  const { exit } = useApp();
+  onMounted(() => {
+    process.stdout.write("__READY__");
+    setTimeout(() => exit(), 100);
+  });
+  return () => h(Text, null, () => "hello");
+});
+
+const app = createApp(App);
+app.mount({ rawMode: "auto", exitOnCtrlC: false });
+await app.waitUntilExit();
+console.log("exited");

packages/runtime-tests/integration/pty/fixtures/cursor-usecursor-app.tsx

@@ -0,0 +1,27 @@
+import process from "node:process";
+import { Box, Text, createApp, useApp, useCursor } from "@vue-tui/runtime";
+import { defineComponent, h, onMounted } from "vue";
+
+// A useCursor app. log-update hides-then-shows the cursor within a single
+// render(): it lazily hides at the top, then emits the cursor SHOW + cursorTo
+// suffix for the active position. So the LAST cursor visibility change on the
+// first frame must be a SHOW (cursor visible at the requested position), with no
+// trailing re-hide — exactly Ink's ordering, and unchanged by removing the eager
+// mount-time hide.
+const App = defineComponent(() => {
+  const { exit } = useApp();
+  const { setCursorPosition } = useCursor();
+  onMounted(() => {
+    process.stdout.write("__READY__");
+    setTimeout(() => exit(), 100);
+  });
+  return () => {
+    setCursorPosition({ x: 2, y: 0 });
+    return h(Box, null, () => h(Text, null, () => "> "));
+  };
+});
+
+const app = createApp(App);
+app.mount({ rawMode: "auto", exitOnCtrlC: false });
+await app.waitUntilExit();
+console.log("exited");

packages/runtime/src/render.ts

@@ -788,11 +788,17 @@ export function createApp(root: Component, rootProps?: RootProps | null): TuiApp
         // wrapper and the "should we touch log-update at all" decision separate:
         //
         //  - Outer gate (ink.tsx:1094 `output !== lastOutput || log.isCursorDirty()`):
-        //    decides whether to call the (throttled) log at all. A cursor-only move
-        //    whose position is unchanged from the previous render is still dirty, so
-        //    it must reach log-update — willRender() alone would miss it because it
-        //    compares positions, not the dirty flag. Here that gate is `willRender ||
-        //    isCursorDirty`; when both are false we skip the write entirely.
+        //    decides whether to call the (throttled) log at all. It compares the RAW
+        //    frame (`output`, no trailing "\n") against the PREVIOUS frame
+        //    (frameState.lastOutput, set at the end of this fn) — NOT log-update's
+        //    \n-suffixed previousOutput. This is load-bearing for the empty-frame
+        //    case: on the first commit of an app that renders nothing, both are ""
+        //    so the gate is false and log-update — including its LAZY cursor hide —
+        //    is never reached, so an empty app emits zero cursor escapes (cursor
+        //    stays visible), matching Ink. Using willRender(outputToRender) here
+        //    instead would compare "\n" against "" and wrongly fire the hide. A
+        //    cursor-only move whose position is unchanged is still dirty, so the
+        //    `|| isCursorDirty` disjunct keeps it reaching log-update.
         //  - Inner gate (ink.tsx:372-382, inside throttledLog): wraps the write in
         //    BSU/ESU only when `willRender(output)` is true. The cursor-dirty-but-not-
         //    willRender case calls log-update WITHOUT the BSU/ESU wrapper, so the dirty
@@ -804,7 +810,7 @@ export function createApp(root: Component, rootProps?: RootProps | null): TuiApp
         // afterwards would be stale. Both reads are pure (no mutation), and the
         // bsu/esu wrapper is gated on this single pre-write snapshot.
         const willRender = writer.willRender(outputToRender);
-        if (willRender || writer.isCursorDirty()) {
+        if (output !== frameState.lastOutput || writer.isCursorDirty()) {
           const shouldWrap = synchronize && willRender;
           if (shouldWrap) stdout.write(bsu);
           writer.write(outputToRender);
@@ -1050,32 +1056,26 @@ export function createApp(root: Component, rootProps?: RootProps | null): TuiApp
     }
     mountedAlternateScreen = alternateScreen;
 
-    // Hide cursor on mount (matching Ink). Only in interactive mode — in
-    // debug/test mode or non-interactive the stream may not be a real TTY.
-    // Screen-reader mode leaves the cursor VISIBLE (Ink parity G59): Ink's SR
-    // path never hides the cursor (the dedicated SR write branch above does no
-    // cursor management), so a screen-reader user keeps a real terminal cursor.
+    // No eager mount-time cursor hide here (matching Ink). Ink hides the cursor
+    // LAZILY: the non-alt-screen hide comes from log-update's isTTY-gated
+    // cliCursor.hide on the first render that actually writes (log-update.ts:
+    // 55-59), and the onRender outer gate skips log-update entirely for an empty
+    // frame (ink.tsx:1094 `output !== lastOutput`, both "" on the first empty
+    // commit). So an interactive app whose root renders nothing emits ZERO
+    // cursor escapes — the cursor stays visible — while a non-empty / useCursor
+    // app hides on its first render via the same lazy path. The renderInteractive
+    // commit gate below mirrors that `output !== frameState.lastOutput` outer
+    // condition so the empty-frame skip (and thus the no-hide behavior) holds.
     //
-    // This MUST happen BEFORE originalMount: mounting flushes Vue synchronously
-    // and the first commit (which, when useCursor() is active, ends with a
-    // showCursor + cursorTo via log-update) runs inside originalMount via a
-    // post-flush callback. Writing the hide afterwards would land AFTER that
-    // show and leave the cursor hidden — the last visibility change must be the
-    // show, mirroring Ink, which hides before its first render, not after.
-    // isTTY gate (cli-cursor short-circuit, cli-cursor/index.js:8-24): cursor
-    // hide/show is a TTY-only concern. In Ink the only mount-time hide lives in
-    // setAlternateScreen (alt-screen + isTTY gated); the non-alt-screen hide
-    // comes from log-update's isTTY-gated cliCursor.hide. So a caller forcing
-    // interactive onto a piped/non-TTY stdout must NOT leak a hide here.
-    if (
-      !debug &&
-      interactive &&
-      !mountedAlternateScreen &&
-      !isScreenReaderEnabled &&
-      Boolean(stdout.isTTY)
-    ) {
-      stdout.write("\x1b[?25l");
-    }
+    // Ordering for a useCursor app is preserved without an eager hide: log-update
+    // hides-then-shows WITHIN a single render() (it hides at the top, then emits
+    // the showCursor + cursorTo suffix for the active position), so the last
+    // visibility change on the first frame is the SHOW — exactly Ink's ordering.
+    //
+    // Screen-reader mode leaves the cursor VISIBLE (Ink parity G59): its
+    // dedicated write branch never routes through log-update, so no hide. The
+    // only mount-time hide that remains is the alt-screen one above
+    // (setAlternateScreen, alt-screen + isTTY gated), mirroring Ink.
 
     // The cursor (and alternate screen) have already been hidden/entered above,
     // but the process-exit and signal-exit teardown handlers are not wired until