Split diff horizontal scroll into platform-specific components

Use Metro .web.tsx resolution to separate native gesture arbitration
(RNGH ScrollView, waitFor, closeGestureRef) from web (plain RN ScrollView),
fixing text selection drag on desktop web.

packages/app/src/components/git-diff-pane.tsx

@@ -2,7 +2,6 @@ import {
   useState,
   useCallback,
   useEffect,
-  useId,
   useMemo,
   useRef,
   memo,
@@ -20,7 +19,6 @@ import {
   type NativeSyntheticEvent,
   type NativeScrollEvent,
 } from "react-native";
-import { ScrollView, type ScrollView as ScrollViewType } from "react-native-gesture-handler";
 import { StyleSheet, UnistylesRuntime, useUnistyles } from "react-native-unistyles";
 import AsyncStorage from "@react-native-async-storage/async-storage";
 import {
@@ -44,8 +42,7 @@ import {
 } from "@/hooks/use-checkout-diff-query";
 import { useCheckoutStatusQuery } from "@/hooks/use-checkout-status-query";
 import { useCheckoutPrStatusQuery } from "@/hooks/use-checkout-pr-status-query";
-import { useHorizontalScrollOptional } from "@/contexts/horizontal-scroll-context";
-import { useExplorerSidebarAnimation } from "@/contexts/explorer-sidebar-animation-context";
+import { DiffScroll } from "./diff-scroll";
 import {
   darkHighlightColors,
   lightHighlightColors,
@@ -262,41 +259,6 @@ function DiffFileBody({
   testID?: string;
 }) {
   const [scrollViewWidth, setScrollViewWidth] = useState(0);
-  const [isAtLeftEdge, setIsAtLeftEdge] = useState(true);
-  const horizontalScroll = useHorizontalScrollOptional();
-  const scrollId = useId();
-  const scrollViewRef = useRef<ScrollViewType>(null);
-
-  // Get the close gesture ref from animation context (may not be available outside sidebar)
-  let closeGestureRef: React.MutableRefObject<any> | undefined;
-  try {
-    const animation = useExplorerSidebarAnimation();
-    closeGestureRef = animation.closeGestureRef;
-  } catch {
-    // Not inside ExplorerSidebarAnimationProvider, which is fine
-  }
-
-  // Register/unregister scroll offset tracking
-  useEffect(() => {
-    if (!horizontalScroll) return;
-    // Start at 0 (not scrolled)
-    horizontalScroll.registerScrollOffset(scrollId, 0);
-    return () => {
-      horizontalScroll.unregisterScrollOffset(scrollId);
-    };
-  }, [horizontalScroll, scrollId]);
-
-  const handleScroll = useCallback(
-    (event: NativeSyntheticEvent<NativeScrollEvent>) => {
-      const offsetX = event.nativeEvent.contentOffset.x;
-      // Track if we're at the left edge (with small threshold for float precision)
-      setIsAtLeftEdge(offsetX <= 1);
-      if (horizontalScroll) {
-        horizontalScroll.registerScrollOffset(scrollId, offsetX);
-      }
-    },
-    [horizontalScroll, scrollId],
-  );
 
   return (
     <View
@@ -364,29 +326,18 @@ function DiffFileBody({
         }
 
         return (
-          <ScrollView
-            ref={scrollViewRef}
-            horizontal
-            nestedScrollEnabled
-            showsHorizontalScrollIndicator
-            bounces={false}
+          <DiffScroll
+            scrollViewWidth={scrollViewWidth}
+            onScrollViewWidthChange={setScrollViewWidth}
             style={styles.diffContent}
             contentContainerStyle={styles.diffContentInner}
-            onScroll={handleScroll}
-            scrollEventThrottle={16}
-            onLayout={(e) => setScrollViewWidth(e.nativeEvent.layout.width)}
-            // When at left edge, wait for close gesture to fail before scrolling.
-            // The close gesture fails quickly on leftward swipes (failOffsetX=-10),
-            // so scrolling left works normally. On rightward swipes, close gesture
-            // activates and closes the sidebar.
-            waitFor={isAtLeftEdge && closeGestureRef?.current ? closeGestureRef : undefined}
           >
             <View
               style={[styles.linesContainer, scrollViewWidth > 0 && { minWidth: scrollViewWidth }]}
             >
               {linesContent}
             </View>
-          </ScrollView>
+          </DiffScroll>
         );
       })()}
     </View>