fix(tui): clear stale composer placeholder

tui/src/app.test.ts

@@ -137,6 +137,30 @@ describe("NanobotTui layout", () => {
     expect(sent).toEqual(["你好"])
   })
 
+  test("clears the placeholder on the first typed character", async () => {
+    setup = await createRenderer({ width: 72, height: 20, screenMode: "alternate-screen" })
+    const app = mount(setup)
+    const composer = (app as unknown as { composer: TextareaRenderable }).composer
+    await setup.renderOnce()
+    expect(setup.captureCharFrame()).toContain("Ask nanobot anything")
+
+    setup.mockInput.typeText("bu")
+    await setup.flush()
+    const frame = setup.captureCharFrame()
+
+    expect(composer.plainText).toBe("bu")
+    expect(composer.placeholder).toBeNull()
+    expect(frame).toContain("bu")
+    expect(frame).not.toContain("Ask nanobot anything")
+    expect(frame).not.toContain("buAsk nanobot anything")
+
+    setup.mockInput.pressBackspace()
+    setup.mockInput.pressBackspace()
+    await setup.flush()
+    expect(composer.placeholder).toBe("Ask nanobot anything")
+    expect(setup.captureCharFrame()).toContain("Ask nanobot anything")
+  })
+
   test("recalls submitted prompts without stealing multiline cursor movement", async () => {
     const sent: string[] = []
     setup = await createRenderer({ width: 72, height: 20, screenMode: "alternate-screen" })

tui/src/app.ts

@@ -82,6 +82,8 @@ const LIGHT: Palette = {
   cool: "#0F766E",
 }
 
+const COMPOSER_PLACEHOLDER = "Ask nanobot anything"
+
 function syntaxStyle(palette: Palette): SyntaxStyle {
   const color = (value: string) => {
     const parsed = RGBA.fromHex(value)
@@ -231,7 +233,7 @@ export class NanobotTui {
       minHeight: 1,
       maxHeight: 8,
       wrapMode: "word",
-      placeholder: "Ask nanobot anything",
+      placeholder: COMPOSER_PLACEHOLDER,
       placeholderColor: this.palette.faint,
       textColor: this.palette.text,
       focusedTextColor: this.palette.text,
@@ -243,7 +245,10 @@ export class NanobotTui {
         { name: "return", action: "submit" },
         { name: "return", meta: true, action: "newline" },
       ],
-      onContentChange: () => this.resizeComposer(),
+      onContentChange: () => {
+        this.syncComposerPlaceholder()
+        this.resizeComposer()
+      },
       // IMEs may commit their final composed glyph after Enter. Matching the
       // OpenCode/OpenTUI integration, defer twice before reading plainText.
       onSubmit: () => this.deferSubmit(),
@@ -673,6 +678,14 @@ export class NanobotTui {
     this.composerFrame.maxHeight = maxHeight + 2
   }
 
+  private syncComposerPlaceholder(): void {
+    // OpenTUI normally suppresses placeholder glyphs while the editor is not
+    // empty. Explicitly removing them also invalidates their old cells, which
+    // prevents stale placeholder text in differential/embedded terminals.
+    const placeholder = this.composer.plainText ? null : COMPOSER_PLACEHOLDER
+    if (this.composer.placeholder !== placeholder) this.composer.placeholder = placeholder
+  }
+
   private async copySelection(text: string): Promise<void> {
     if (!text) return
     try {