fix(app): restore native composer auto-growth (#3681)

Native multiline inputs were always scrollable, which kept iOS content-size measurement pinned to one line. Keep native sizing intrinsic below the cap and enable internal scrolling only after the composer reaches it.

packages/app/src/composer/input/height-style.test.ts

@@ -0,0 +1,38 @@
+import { describe, expect, it } from "vitest";
+import { resolveComposerInputHeightStyle, shouldScrollComposerInput } from "./height-style";
+
+describe("composer input height style", () => {
+  it("applies the measured height alongside its bounds", () => {
+    expect(
+      resolveComposerInputHeightStyle({
+        inputHeight: 96,
+        minInputHeight: 30,
+        maxInputHeight: 240,
+        applyMeasuredHeight: true,
+      }),
+    ).toEqual({
+      height: 96,
+      minHeight: 30,
+      maxHeight: 240,
+    });
+  });
+
+  it("lets a native multiline input expose its intrinsic content height", () => {
+    expect(
+      resolveComposerInputHeightStyle({
+        inputHeight: 96,
+        minInputHeight: 30,
+        maxInputHeight: 240,
+        applyMeasuredHeight: false,
+      }),
+    ).toEqual({
+      minHeight: 30,
+      maxHeight: 240,
+    });
+  });
+
+  it("keeps native measurement unconstrained until the composer reaches its cap", () => {
+    expect(shouldScrollComposerInput({ inputHeight: 30, maxInputHeight: 240 })).toBe(false);
+    expect(shouldScrollComposerInput({ inputHeight: 240, maxInputHeight: 240 })).toBe(true);
+  });
+});

packages/app/src/composer/input/height-style.ts

@@ -0,0 +1,24 @@
+interface ComposerInputHeightStyleInput {
+  inputHeight: number;
+  minInputHeight: number;
+  maxInputHeight: number;
+  applyMeasuredHeight: boolean;
+}
+
+export function resolveComposerInputHeightStyle(input: ComposerInputHeightStyleInput) {
+  // Native must remain intrinsically sized below the cap so
+  // onContentSizeChange can observe growth. Web gets its height from the DOM
+  // mirror and therefore needs that measurement applied explicitly.
+  return {
+    ...(input.applyMeasuredHeight ? { height: input.inputHeight } : {}),
+    minHeight: input.minInputHeight,
+    maxHeight: input.maxInputHeight,
+  };
+}
+
+export function shouldScrollComposerInput(input: {
+  inputHeight: number;
+  maxInputHeight: number;
+}): boolean {
+  return input.inputHeight >= input.maxInputHeight;
+}

packages/app/src/composer/input/input.tsx

@@ -56,6 +56,7 @@ import { useIsCompactFormFactor } from "@/constants/layout";
 import { useComposerKeyboardScope } from "@/composer/keyboard-scope";
 import { RenderProfile } from "@/utils/render-profiler";
 import { useComposerHeightMirror } from "./height-mirror";
+import { resolveComposerInputHeightStyle, shouldScrollComposerInput } from "./height-style";
 import { resolveComposerInputMode, type ComposerInputMode } from "@/composer/input-mode";
 import type { NativePastedFile } from "@/composer/native-pasted-image";
 import {
@@ -990,20 +991,6 @@ function resolveMaxInputHeight(windowHeight: number): number {
   return Math.max(DEFAULT_MAX_INPUT_HEIGHT, Math.floor(windowHeight * MAX_INPUT_VIEWPORT_RATIO));
 }
 
-function computeTextInputHeightStyle(inputHeight: number, maxInputHeight: number) {
-  if (isWeb) {
-    return {
-      height: inputHeight,
-      minHeight: MIN_INPUT_HEIGHT,
-      maxHeight: maxInputHeight,
-    };
-  }
-  return {
-    minHeight: MIN_INPUT_HEIGHT,
-    maxHeight: maxInputHeight,
-  };
-}
-
 function isTextAreaLike(v: unknown): v is TextAreaHandle {
   return typeof v === "object" && v !== null && "scrollHeight" in v;
 }
@@ -1764,7 +1751,12 @@ export const MessageInput = forwardRef<MessageInputRef, MessageInputProps>(
       () => [
         styles.textInput,
         mode.isMonospace && styles.textInputMonospace,
-        computeTextInputHeightStyle(inputHeight, maxInputHeight),
+        resolveComposerInputHeightStyle({
+          inputHeight,
+          minInputHeight: MIN_INPUT_HEIGHT,
+          maxInputHeight,
+          applyMeasuredHeight: isWeb,
+        }),
       ],
       [inputHeight, maxInputHeight, mode.isMonospace],
     );
@@ -1838,7 +1830,7 @@ export const MessageInput = forwardRef<MessageInputRef, MessageInputProps>(
               onFocus={handleInputFocus}
               onBlur={handleInputBlur}
               editable={!isDictating && !isRealtimeVoiceForCurrentAgent && !disabled}
-              scrollEnabled={isWeb ? inputHeight >= maxInputHeight : true}
+              scrollEnabled={shouldScrollComposerInput({ inputHeight, maxInputHeight })}
               autoFocus={false}
               onContentSizeChange={handleContentSizeChange}
               onKeyPress={shouldHandleWebKeyPress ? handleDesktopKeyPress : undefined}