fix: improve mobile terminal alternate-screen detection and scrolling

- Revert touch-action: none on .xterm-viewport so xterm.js can fall back
  to its own viewport scrolling when the custom handler doesn't take over.
- Detect alternate screen via reference equality
  (term.buffer.active === term.buffer.alternate) instead of the
  string, which could report normal buffer incorrectly.
- Lower vertical-scroll activation threshold from 4px to 2px and only
  prevent default once a vertical gesture is recognized.
- In normal buffer use term.scrollLines() so xterm.js handles the buffer
  scroll consistently; in alternate screen continue sending SGR 1006
  mouse-wheel sequences to tmux/vim.

Quality gates: typecheck, lint clean, npm test -- --run 87 passed.

apps/web/src/components/features/terminal/terminal.tsx

@@ -355,10 +355,9 @@ export const TerminalComponent = React.forwardRef<TerminalRef, TerminalProps>(
 			// In normal mode xterm.js has a scrollable viewport; in alternate
 			// screen (tmux/vim) there is no scrollback and the only way to
 			// scroll is to send mouse-wheel protocol sequences to the
-			// application.  We detect which situation we're in by checking
-			// the active buffer type.  We also lock the browser into the
-			// terminal area: touchstart prevents the browser from starting a
-			// page-scroll gesture, so swipes always go to the terminal.
+			// application.  We detect the active buffer by reference equality
+			// (term.buffer.active === term.buffer.alternate) because the
+			// `type` string can be unreliable in some xterm.js versions.
 			let touchCleanup: (() => void) | undefined;
 			if (isMobile) {
 				let startY = 0;
@@ -388,8 +387,10 @@ export const TerminalComponent = React.forwardRef<TerminalRef, TerminalProps>(
 					scrollPending = 0;
 				};
 
-				const isNormalBuffer = () => {
-					return termRef.current?.buffer.active.type === "normal";
+				const isAlternateScreen = () => {
+					const term = termRef.current;
+					if (!term) return false;
+					return term.buffer.active === term.buffer.alternate;
 				};
 
 				const onTouchStart = (e: TouchEvent) => {
@@ -405,35 +406,36 @@ export const TerminalComponent = React.forwardRef<TerminalRef, TerminalProps>(
 					const touch = e.touches[0];
 					const deltaY = startY - touch.clientY;
 					const deltaX = Math.abs(startX - touch.clientX);
+
+					// Decide early whether this is a vertical scroll gesture.
 					if (!isScrolling) {
-						if (Math.abs(deltaY) > deltaX && Math.abs(deltaY) > 4) {
+						if (Math.abs(deltaY) > deltaX && Math.abs(deltaY) > 2) {
 							isScrolling = true;
 						}
 					}
-					if (isScrolling) {
-						// Always stop the browser from treating this as a page scroll.
-						e.preventDefault();
+					if (!isScrolling) return;
 
-						const normalBuffer = isNormalBuffer();
-						const viewport = container.querySelector(
-							".xterm-viewport",
-						) as HTMLElement | null;
+					// Take over the gesture so the page/toolbar never scrolls.
+					e.preventDefault();
 
-						if (normalBuffer && viewport) {
-							// Normal buffer: scroll the xterm viewport directly.
-							viewport.scrollTop += deltaY;
-						} else {
-							// Alternate screen (tmux/vim): accumulate the swipe and
-							// send SGR 1006 mouse-wheel events in steps.
-							scrollPending += deltaY;
-							flushScroll();
+					if (isAlternateScreen()) {
+						// Alternate screen (tmux/vim): accumulate the swipe and
+						// send SGR 1006 mouse-wheel events in steps.
+						scrollPending += deltaY;
+						flushScroll();
+					} else if (termRef.current) {
+						// Normal buffer: let xterm.js scroll its own viewport by
+						// the number of lines corresponding to the swipe distance.
+						const lineHeight =
+							termRef.current.options.fontSize != null
+								? (termRef.current.options.fontSize as number) * 1.2
+								: 10;
+						const lines = Math.round(deltaY / lineHeight);
+						if (lines !== 0) {
+							termRef.current.scrollLines(lines);
 						}
-						startY = touch.clientY;
-					} else if (Math.abs(deltaY) > 4 || Math.abs(deltaX) > 4) {
-						// Once the user has moved far enough to be considered a
-						// gesture, prevent any default page scroll/pinch behavior.
-						e.preventDefault();
 					}
+					startY = touch.clientY;
 				};
 				const onTouchEnd = () => {
 					if (isScrolling) {