fix Scroll-Drag conflicts + fix dragStartDistanceMotionThreshold problem for dropdown drag error + provide 2 public/protected interfaces to Xun Ye

Runtime/gestures/monodrag.cs

                     });
                 }
 
-                this.invokeCallback<object>("onEnd", () => {
-                        this.onEnd(new DragEndDetails(
-                            velocity: Velocity.zero,
-                            primaryVelocity: 0.0f
-                        ));
-                        return null;
-                    }, debugReport: () => { return "Pointer scroll end"; }
-                );
-
-
-                this._state = _DragState.ready;
+                this.stopTrackingScrollerPointer(evt.pointer);
                 return;
             }
 
 
         public override void rejectGesture(int pointer) {
             this.stopTrackingPointer(pointer);
+        }
+
+        protected override void didStopTrackingLastScrollerPointer(int pointer) {
+            this._state = _DragState.ready;
+            this.invokeCallback<object>("onEnd", () => {
+                    this.onEnd(new DragEndDetails(
+                        velocity: Velocity.zero,
+                        primaryVelocity: 0.0f
+                    ));
+                    return null;
+                }, debugReport: () => { return "Pointer scroll end"; }
+            );
         }
 
         protected override void didStopTrackingLastPointer(int pointer) {

Runtime/gestures/recognizer.cs

 
         protected abstract void didStopTrackingLastPointer(int pointer);
 
+        protected virtual void didStopTrackingLastScrollerPointer(int pointer) {
+        }
+
         protected virtual void resolve(GestureDisposition disposition) {
             var localEntries = new List<GestureArenaEntry>(this._entries.Values);
             this._entries.Clear();
 
         protected void startTrackingScrollerPointer(int pointer) {
             GestureBinding.instance.pointerRouter.addRoute(pointer, this.handleEvent);
+        }
+
+        protected void stopTrackingScrollerPointer(int pointer) {
+            if (this._trackedPointers.isEmpty()) {
+                this.didStopTrackingLastScrollerPointer(pointer);
+            }
         }
 
         protected void startTrackingPointer(int pointer) {

Runtime/rendering/paragraph.cs

 using Unity.UIWidgets.painting;
 using Unity.UIWidgets.ui;
 using UnityEngine;
+using Rect = Unity.UIWidgets.ui.Rect;
 
 namespace Unity.UIWidgets.rendering {
     public enum TextOverflow {
                 this._textPainter.textDirection = this.textDirection;
                 this.markNeedsLayout();
             }
+        }
+
+        protected Offset getOffsetForCaret(TextPosition position, Rect caretPrototype) {
+            D.assert(this._textPainter != null);
+            return this._textPainter.getOffsetForCaret(position, caretPrototype);
         }
 
         public bool softWrap {

Runtime/widgets/media_query.cs

             return of(context, nullOk: true)?.textScaleFactor ?? 1.0f;
         }
 
-        static bool boldTextOverride(BuildContext context) {
+        public static bool boldTextOverride(BuildContext context) {
             return of(context, nullOk: true)?.boldText ?? false;
         }
 

Runtime/widgets/scroll_physics.cs

             }
         }
 
-        public virtual float dragStartDistanceMotionThreshold {
+        public virtual float? dragStartDistanceMotionThreshold {
-                if (this.parent == null) {
-                    return 0.0f;
-                }
-
-                return this.parent.dragStartDistanceMotionThreshold;
+                return this.parent?.dragStartDistanceMotionThreshold;
             }
         }
 
                    Mathf.Min(0.000816f * Mathf.Pow(existingVelocity.abs(), 1.967f), 40000.0f);
         }
 
-        public override float dragStartDistanceMotionThreshold {
+        public override float? dragStartDistanceMotionThreshold {
             get { return 3.5f; }
         }
     }