fix(maestro): reliable touch scrollback on mobile

xterm's native touch scrolling crawled a line at a time and stalled after a
few. Map finger drags on the viewport to term.scrollLines (skipped on the alt
screen, which the app owns) and claim vertical pans via touch-action: pan-y.

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

web/src/attach.ts

@@ -130,6 +130,51 @@ export function createAttachConnection(
   term.loadAddon(fit);
   term.open(body);
 
+  // Touch scrollback: xterm's built-in touch handling on the viewport is jittery
+  // (drags crawl a line at a time and stall after a few), so map finger drags to
+  // term.scrollLines ourselves. Skipped on the alternate screen (full-screen
+  // TUIs like tmux/Claude Code) — there's no scrollback to move and the app owns
+  // the gesture. `touch-action: pan-y` (CSS) tells the browser we handle Y.
+  {
+    const vp = el.querySelector(".xterm-viewport") as HTMLElement | null;
+    const cellH = () => {
+      const core = (term as unknown as {
+        _core?: { _renderService?: { dimensions?: { css?: { cell?: { height: number } } } } };
+      })._core;
+      return core?._renderService?.dimensions?.css?.cell?.height || 16;
+    };
+    if (vp) {
+      let lastY = 0;
+      let remainder = 0;
+      vp.addEventListener(
+        "touchstart",
+        (e) => {
+          if (e.touches.length !== 1) return;
+          lastY = e.touches[0].clientY;
+          remainder = 0;
+        },
+        { passive: true },
+      );
+      vp.addEventListener(
+        "touchmove",
+        (e) => {
+          if (e.touches.length !== 1) return;
+          if (term.buffer.active.type === "alternate") return; // app-owned
+          const y = e.touches[0].clientY;
+          const px = lastY - y + remainder; // finger up → scroll toward newer
+          lastY = y;
+          const lines = Math.trunc(px / cellH());
+          remainder = px - lines * cellH();
+          if (lines !== 0) {
+            term.scrollLines(lines);
+            e.preventDefault();
+          }
+        },
+        { passive: false },
+      );
+    }
+  }
+
   // Mobile: the special-key bar is only useful while the soft keyboard is up,
   // so reveal it only when the terminal's hidden textarea is focused (CSS still
   // gates it to coarse pointers). Tapping a keycap keeps focus via pointerdown

web/src/styles.css

@@ -1148,6 +1148,9 @@ textarea {
 .attach-body .xterm .xterm-viewport {
   overflow-y: auto;
   scrollbar-width: none;
+  /* We drive scrollback from touchmove (see attach.ts); claim vertical pans so
+     the browser doesn't fight the handler or trigger gestures mid-scroll. */
+  touch-action: pan-y;
 }
 
 .attach-body .xterm .xterm-viewport::-webkit-scrollbar {