fix(app): composer textarea shrinks back down on web

Measuring scrollHeight on the live textarea with an explicit height set can only grow — it can never report a shorter content height. The old `height: auto` toggle flickered across browsers. Use a hidden off-DOM textarea that mirrors the composer's text-metric styles and width, and read its scrollHeight on value/resize changes. Gated by Metro file extension (.web / .native) so it's trivial to rip out later.
2026-04-19 14:26:50 +07:00
parent fbc2371b95
commit 2238171955
4 changed files with 131 additions and 66 deletions
@@ -0,0 +1,106 @@
+import { useCallback, useEffect, useLayoutEffect, useRef } from "react";
+import type { RefObject } from "react";
+
+interface Args {
+  value: string;
+  textareaRef: RefObject<HTMLElement | null>;
+  minHeight: number;
+  maxHeight: number;
+  onHeight: (height: number) => void;
+}
+
+const COPIED_STYLES = [
+  "fontFamily",
+  "fontSize",
+  "fontWeight",
+  "fontStyle",
+  "fontVariant",
+  "lineHeight",
+  "letterSpacing",
+  "wordSpacing",
+  "textTransform",
+  "textIndent",
+  "whiteSpace",
+  "wordWrap",
+  "overflowWrap",
+  "wordBreak",
+  "tabSize",
+  "paddingTop",
+  "paddingRight",
+  "paddingBottom",
+  "paddingLeft",
+] as const;
+
+export function useComposerHeightMirror({
+  value,
+  textareaRef,
+  minHeight,
+  maxHeight,
+  onHeight,
+}: Args): void {
+  const paramsRef = useRef({ value, minHeight, maxHeight, onHeight });
+  paramsRef.current = { value, minHeight, maxHeight, onHeight };
+
+  const mirrorRef = useRef<HTMLTextAreaElement | null>(null);
+
+  useEffect(() => {
+    if (typeof document === "undefined") return;
+    const mirror = document.createElement("textarea");
+    mirror.setAttribute("aria-hidden", "true");
+    mirror.setAttribute("tabindex", "-1");
+    mirror.readOnly = true;
+    mirror.rows = 1;
+    const style = mirror.style;
+    style.position = "absolute";
+    style.top = "0";
+    style.left = "0";
+    style.visibility = "hidden";
+    style.pointerEvents = "none";
+    style.overflow = "hidden";
+    style.border = "0";
+    style.margin = "0";
+    style.resize = "none";
+    style.zIndex = "-1";
+    style.boxSizing = "border-box";
+    document.body.appendChild(mirror);
+    mirrorRef.current = mirror;
+    return () => {
+      mirror.remove();
+      mirrorRef.current = null;
+    };
+  }, []);
+
+  const measure = useCallback(() => {
+    const mirror = mirrorRef.current;
+    const source = textareaRef.current;
+    if (!mirror || !source || typeof window === "undefined") return;
+    if (!(source instanceof HTMLElement)) return;
+
+    const cs = window.getComputedStyle(source);
+    const ms = mirror.style;
+    for (const prop of COPIED_STYLES) {
+      ms[prop] = cs[prop];
+    }
+    ms.width = `${source.clientWidth}px`;
+
+    const { value, minHeight, maxHeight, onHeight } = paramsRef.current;
+    // Trailing newline is collapsed by textarea measurement — pad with a space.
+    mirror.value = value.endsWith("\n") ? `${value} ` : value;
+
+    const next = Math.max(minHeight, Math.min(maxHeight, mirror.scrollHeight));
+    onHeight(next);
+  }, [textareaRef]);
+
+  useLayoutEffect(() => {
+    measure();
+  }, [value, measure]);
+
+  useEffect(() => {
+    const source = textareaRef.current;
+    if (!source || !(source instanceof HTMLElement)) return;
+    if (typeof ResizeObserver === "undefined") return;
+    const observer = new ResizeObserver(() => measure());
+    observer.observe(source);
+    return () => observer.disconnect();
+  }, [textareaRef, measure]);
+}