fix(app): stop composer height flicker

Native scrollbar appearance changed the textarea's usable width, feeding alternating measurements back into the composer height mirror. Reserve the gutter so wrapping and height remain stable.

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

@@ -74,6 +74,7 @@ import {
 } from "./state";
 
 const DEFAULT_SEND_KEYS: ShortcutKey[][] = [["Enter"]];
+const COMPOSER_INPUT_DATASET = { composerInput: "" } as const;
 
 export interface AttachmentMenuItem {
   id: string;
@@ -1807,6 +1808,7 @@ export const MessageInput = forwardRef<MessageInputRef, MessageInputProps>(
           <View style={styles.textInputScrollWrapper}>
             <ThemedTextInput
               ref={textInputRef}
+              dataSet={COMPOSER_INPUT_DATASET}
               value={value}
               onChangeText={handleInputChange}
               placeholder={placeholder ?? t("composer.placeholders.fallback")}

packages/app/src/styles/install-web-scrollbar-styles.web.ts

@@ -19,6 +19,10 @@ export function installWebScrollbarStyles(): () => void {
   scrollbar-width: ${WEB_SCROLLBAR_WIDTH};
 }
 
+[data-composer-input] {
+  scrollbar-gutter: stable;
+}
+
 *::-webkit-scrollbar {
   width: ${WEB_SCROLLBAR_SIZE_PX}px;
   height: ${WEB_SCROLLBAR_SIZE_PX}px;