vendor: import KillerPDF 1.7.5 source (GPL-3.0) as the base for MMD PDF

Services/WheelPageFlipGate.cs

@@ -0,0 +1,55 @@
+namespace KillerPDF.Services
+{
+    /// <summary>
+    /// Separates fast in-page wheel scrolling from page navigation at the edge. Momentum events
+    /// immediately following a content scroll are ignored; after that, two standard wheel notches
+    /// in the same direction confirm that the user intends to change pages (#205).
+    /// </summary>
+    internal sealed class WheelPageFlipGate
+    {
+        private static readonly TimeSpan MomentumQuietPeriod = TimeSpan.FromMilliseconds(250);
+        private static readonly TimeSpan ConfirmationWindow = TimeSpan.FromMilliseconds(650);
+        private const int ConfirmationDelta = 240;
+
+        private DateTime _blockUntilUtc;
+        private DateTime _lastEdgeWheelUtc;
+        private int _direction;
+        private int _accumulatedDelta;
+
+        internal void NoteContentScroll(DateTime nowUtc)
+        {
+            _blockUntilUtc = nowUtc + MomentumQuietPeriod;
+            ResetConfirmation();
+        }
+
+        internal bool TryConfirm(int delta, DateTime nowUtc)
+        {
+            if (delta == 0 || nowUtc < _blockUntilUtc)
+            {
+                ResetConfirmation();
+                return false;
+            }
+
+            int direction = Math.Sign(delta);
+            if (_direction != direction || nowUtc - _lastEdgeWheelUtc > ConfirmationWindow)
+            {
+                _direction = direction;
+                _accumulatedDelta = 0;
+            }
+
+            _lastEdgeWheelUtc = nowUtc;
+            _accumulatedDelta += Math.Abs(delta);
+            if (_accumulatedDelta < ConfirmationDelta) return false;
+
+            ResetConfirmation();
+            return true;
+        }
+
+        private void ResetConfirmation()
+        {
+            _lastEdgeWheelUtc = default;
+            _direction = 0;
+            _accumulatedDelta = 0;
+        }
+    }
+}