Text editing shift + tap + drag interaction (#95213)

Supports the desktop text editing interaction of holding shift, tapping the field, and dragging to modify the selection.

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

@@ -1026,9 +1026,23 @@ class TextSelectionGestureDetectorBuilder {
   // The viewport offset pixels of the [RenderEditable] at the last drag start.
   double _dragStartViewportOffset = 0.0;
 
+  // Returns true iff either shift key is currently down.
+  bool get _isShiftPressed {
+    return HardwareKeyboard.instance.logicalKeysPressed
+      .any(<LogicalKeyboardKey>{
+        LogicalKeyboardKey.shiftLeft,
+        LogicalKeyboardKey.shiftRight,
+      }.contains);
+  }
+
   // True iff a tap + shift has been detected but the tap has not yet come up.
   bool _isShiftTapping = false;
 
+  // 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.
+  TextSelection? _shiftTapDragSelection;
+
   /// Handler for [TextSelectionGestureDetector.onTapDown].
   ///
   /// By default, it forwards the tap to [RenderEditable.handleTapDown] and sets
@@ -1050,12 +1064,7 @@ class TextSelectionGestureDetectorBuilder {
       || kind == PointerDeviceKind.stylus;
 
     // Handle shift + click selection if needed.
-    final bool isShiftPressed = HardwareKeyboard.instance.logicalKeysPressed
-        .any(<LogicalKeyboardKey>{
-          LogicalKeyboardKey.shiftLeft,
-          LogicalKeyboardKey.shiftRight,
-        }.contains);
-    if (isShiftPressed && renderEditable.selection?.baseOffset != null) {
+    if (_isShiftPressed && renderEditable.selection?.baseOffset != null) {
       _isShiftTapping = true;
       switch (defaultTargetPlatform) {
         case TargetPlatform.iOS:
@@ -1290,10 +1299,27 @@ class TextSelectionGestureDetectorBuilder {
       || kind == PointerDeviceKind.touch
       || kind == PointerDeviceKind.stylus;
 
-    renderEditable.selectPositionAt(
-      from: details.globalPosition,
-      cause: SelectionChangedCause.drag,
-    );
+    if (_isShiftPressed && renderEditable.selection != null && renderEditable.selection!.isValid) {
+      _isShiftTapping = true;
+      switch (defaultTargetPlatform) {
+        case TargetPlatform.iOS:
+        case TargetPlatform.macOS:
+          _expandSelection(details.globalPosition, SelectionChangedCause.drag);
+          break;
+        case TargetPlatform.android:
+        case TargetPlatform.fuchsia:
+        case TargetPlatform.linux:
+        case TargetPlatform.windows:
+          _extendSelection(details.globalPosition, SelectionChangedCause.drag);
+          break;
+      }
+      _shiftTapDragSelection = renderEditable.selection;
+    } else {
+      renderEditable.selectPositionAt(
+        from: details.globalPosition,
+        cause: SelectionChangedCause.drag,
+      );
+    }
 
     _dragStartViewportOffset = renderEditable.offset.pixels;
   }
@@ -1312,28 +1338,77 @@ class TextSelectionGestureDetectorBuilder {
     if (!delegate.selectionEnabled)
       return;
 
-    // Adjust the drag start offset for possible viewport offset changes.
-    final Offset startOffset = renderEditable.maxLines == 1
-        ? Offset(renderEditable.offset.pixels - _dragStartViewportOffset, 0.0)
-        : Offset(0.0, renderEditable.offset.pixels - _dragStartViewportOffset);
+    if (!_isShiftTapping) {
+      // Adjust the drag start offset for possible viewport offset changes.
+      final Offset startOffset = renderEditable.maxLines == 1
+          ? Offset(renderEditable.offset.pixels - _dragStartViewportOffset, 0.0)
+          : Offset(0.0, renderEditable.offset.pixels - _dragStartViewportOffset);
 
-    renderEditable.selectPositionAt(
-      from: startDetails.globalPosition - startOffset,
-      to: updateDetails.globalPosition,
-      cause: SelectionChangedCause.drag,
-    );
+      return renderEditable.selectPositionAt(
+        from: startDetails.globalPosition - startOffset,
+        to: updateDetails.globalPosition,
+        cause: SelectionChangedCause.drag,
+      );
+    }
+
+    if (_shiftTapDragSelection!.isCollapsed
+        || (defaultTargetPlatform != TargetPlatform.iOS
+            && defaultTargetPlatform != TargetPlatform.macOS)) {
+      return _extendSelection(updateDetails.globalPosition, SelectionChangedCause.drag);
+    }
+
+    // If the drag inverts the selection, Mac and iOS revert to the initial
+    // selection.
+    final TextSelection selection = editableText.textEditingValue.selection;
+    final TextPosition nextExtent = renderEditable.getPositionForPoint(updateDetails.globalPosition);
+    final bool isShiftTapDragSelectionForward =
+        _shiftTapDragSelection!.baseOffset < _shiftTapDragSelection!.extentOffset;
+    final bool isInverted = isShiftTapDragSelectionForward
+        ? nextExtent.offset < _shiftTapDragSelection!.baseOffset
+        : nextExtent.offset > _shiftTapDragSelection!.baseOffset;
+    if (isInverted && selection.baseOffset == _shiftTapDragSelection!.baseOffset) {
+      editableText.userUpdateTextEditingValue(
+        editableText.textEditingValue.copyWith(
+          selection: TextSelection(
+            baseOffset: _shiftTapDragSelection!.extentOffset,
+            extentOffset: nextExtent.offset,
+          ),
+        ),
+        SelectionChangedCause.drag,
+      );
+    } else if (!isInverted
+        && nextExtent.offset != _shiftTapDragSelection!.baseOffset
+        && selection.baseOffset != _shiftTapDragSelection!.baseOffset) {
+      editableText.userUpdateTextEditingValue(
+        editableText.textEditingValue.copyWith(
+          selection: TextSelection(
+            baseOffset: _shiftTapDragSelection!.baseOffset,
+            extentOffset: nextExtent.offset,
+          ),
+        ),
+        SelectionChangedCause.drag,
+      );
+    } else {
+      _extendSelection(updateDetails.globalPosition, SelectionChangedCause.drag);
+    }
   }
 
   /// Handler for [TextSelectionGestureDetector.onDragSelectionEnd].
   ///
-  /// By default, it services as place holder to enable subclass override.
+  /// By default, it simply cleans up the state used for handling certain
+  /// built-in behaviors.
   ///
   /// See also:
   ///
   ///  * [TextSelectionGestureDetector.onDragSelectionEnd], which triggers this
   ///    callback.
   @protected
-  void onDragSelectionEnd(DragEndDetails details) {/* Subclass should override this method if needed. */}
+  void onDragSelectionEnd(DragEndDetails details) {
+    if (_isShiftTapping) {
+      _isShiftTapping = false;
+      _shiftTapDragSelection = null;
+    }
+  }
 
   /// Returns a [TextSelectionGestureDetector] configured with the handlers
   /// provided by this builder.