Fix text selection edge scrolling when inside a horizontal scrollable (#140250)

Fixes #129590

* Consider `AxisDirection` when calculating scroll offset used in determining TextSelection during a drag/long press drag. Previously it seems that we were assuming the direction was always vertical https://github.com/flutter/flutter/blob/30cc83198544582b858e48c7bb9d761ecdb3d944/packages/flutter/lib/src/widgets/text_selection.dart#L2842-L2844 .
* SelectableText now considers RenderEditable offset changes and Scrollable offset changes when calculating the TextSelection during a long press drag.

packages/flutter/lib/src/material/selectable_text.dart

@@ -59,6 +59,31 @@ class _SelectableTextSelectionGestureDetectorBuilder extends TextSelectionGestur
 
   final _SelectableTextState _state;
 
+  /// The viewport offset pixels of any [Scrollable] containing the
+  /// [RenderEditable] at the last drag start.
+  double _dragStartScrollOffset = 0.0;
+
+  /// The viewport offset pixels of the [RenderEditable] at the last drag start.
+  double _dragStartViewportOffset = 0.0;
+
+  double get _scrollPosition {
+    final ScrollableState? scrollableState =
+        delegate.editableTextKey.currentContext == null
+            ? null
+            : Scrollable.maybeOf(delegate.editableTextKey.currentContext!);
+    return scrollableState == null
+        ? 0.0
+        : scrollableState.position.pixels;
+  }
+
+  AxisDirection? get _scrollDirection {
+    final ScrollableState? scrollableState =
+        delegate.editableTextKey.currentContext == null
+            ? null
+            : Scrollable.maybeOf(delegate.editableTextKey.currentContext!);
+    return scrollableState?.axisDirection;
+  }
+
   @override
   void onForcePressStart(ForcePressDetails details) {
     super.onForcePressStart(details);
@@ -72,15 +97,37 @@ class _SelectableTextSelectionGestureDetectorBuilder extends TextSelectionGestur
     // Not required.
   }
 
+  @override
+  void onSingleLongTapStart(LongPressStartDetails details) {
+    if (!delegate.selectionEnabled) {
+      return;
+    }
+    renderEditable.selectWord(cause: SelectionChangedCause.longPress);
+    Feedback.forLongPress(_state.context);
+    _dragStartViewportOffset = renderEditable.offset.pixels;
+    _dragStartScrollOffset = _scrollPosition;
+  }
+
   @override
   void onSingleLongTapMoveUpdate(LongPressMoveUpdateDetails details) {
-    if (delegate.selectionEnabled) {
-      renderEditable.selectWordsInRange(
-        from: details.globalPosition - details.offsetFromOrigin,
-        to: details.globalPosition,
-        cause: SelectionChangedCause.longPress,
-      );
+    if (!delegate.selectionEnabled) {
+      return;
     }
+    // Adjust the drag start offset for possible viewport offset changes.
+    final Offset editableOffset = renderEditable.maxLines == 1
+        ? Offset(renderEditable.offset.pixels - _dragStartViewportOffset, 0.0)
+        : Offset(0.0, renderEditable.offset.pixels - _dragStartViewportOffset);
+    final double effectiveScrollPosition = _scrollPosition - _dragStartScrollOffset;
+    final bool scrollingOnVerticalAxis = _scrollDirection == AxisDirection.up || _scrollDirection == AxisDirection.down;
+    final Offset scrollableOffset = Offset(
+      !scrollingOnVerticalAxis ? effectiveScrollPosition : 0.0,
+      scrollingOnVerticalAxis ? effectiveScrollPosition : 0.0,
+    );
+    renderEditable.selectWordsInRange(
+      from: details.globalPosition - details.offsetFromOrigin - editableOffset - scrollableOffset,
+      to: details.globalPosition,
+      cause: SelectionChangedCause.longPress,
+    );
   }
 
   @override
@@ -100,14 +147,6 @@ class _SelectableTextSelectionGestureDetectorBuilder extends TextSelectionGestur
     }
     _state.widget.onTap?.call();
   }
-
-  @override
-  void onSingleLongTapStart(LongPressStartDetails details) {
-    if (delegate.selectionEnabled) {
-      renderEditable.selectWord(cause: SelectionChangedCause.longPress);
-      Feedback.forLongPress(_state.context);
-    }
-  }
 }
 
 /// A run of selectable text with a single style.

packages/flutter/lib/src/widgets/text_selection.dart

@@ -2131,6 +2131,14 @@ class TextSelectionGestureDetectorBuilder {
         : scrollableState.position.pixels;
   }
 
+  AxisDirection? get _scrollDirection {
+    final ScrollableState? scrollableState =
+        delegate.editableTextKey.currentContext == null
+            ? null
+            : Scrollable.maybeOf(delegate.editableTextKey.currentContext!);
+    return scrollableState?.axisDirection;
+  }
+
   // For a shift + tap + drag gesture, the TextSelection at the point of the
   // tap. Mac uses this value to reset to the original selection when an
   // inversion of the base and offset happens.
@@ -2498,9 +2506,11 @@ class TextSelectionGestureDetectorBuilder {
       final Offset editableOffset = renderEditable.maxLines == 1
           ? Offset(renderEditable.offset.pixels - _dragStartViewportOffset, 0.0)
           : Offset(0.0, renderEditable.offset.pixels - _dragStartViewportOffset);
+      final double effectiveScrollPosition = _scrollPosition - _dragStartScrollOffset;
+      final bool scrollingOnVerticalAxis = _scrollDirection == AxisDirection.up || _scrollDirection == AxisDirection.down;
       final Offset scrollableOffset = Offset(
-        0.0,
-        _scrollPosition - _dragStartScrollOffset,
+        !scrollingOnVerticalAxis ? effectiveScrollPosition : 0.0,
+        scrollingOnVerticalAxis ? effectiveScrollPosition : 0.0,
       );
       switch (defaultTargetPlatform) {
         case TargetPlatform.iOS:
@@ -2839,9 +2849,11 @@ class TextSelectionGestureDetectorBuilder {
       final Offset editableOffset = renderEditable.maxLines == 1
           ? Offset(renderEditable.offset.pixels - _dragStartViewportOffset, 0.0)
           : Offset(0.0, renderEditable.offset.pixels - _dragStartViewportOffset);
+      final double effectiveScrollPosition = _scrollPosition - _dragStartScrollOffset;
+      final bool scrollingOnVerticalAxis = _scrollDirection == AxisDirection.up || _scrollDirection == AxisDirection.down;
       final Offset scrollableOffset = Offset(
-        0.0,
-        _scrollPosition - _dragStartScrollOffset,
+        !scrollingOnVerticalAxis ? effectiveScrollPosition : 0.0,
+        scrollingOnVerticalAxis ? effectiveScrollPosition : 0.0,
       );
       final Offset dragStartGlobalPosition = details.globalPosition - details.offsetFromOrigin;