Fix UI Controls layout auto-resizing to always use pivot-relative logic

#3031

Source/Engine/UI/GUI/Common/Label.cs

@@ -337,9 +337,7 @@ namespace FlaxEngine.GUI
                             size.X = _textSize.X + Margin.Width;
                         if (_autoHeight)
                             size.Y = _textSize.Y + Margin.Height;
-                        var pivotRelative = PivotRelative;
-                        Size = size;
-                        PivotRelative = pivotRelative;
+                        Resize(ref size);
                     }
                 }
             }

Source/Engine/UI/GUI/Control.Bounds.cs

@@ -419,6 +419,19 @@ namespace FlaxEngine.GUI
             }
         }
 
+        /// <summary>
+        /// Resizes the control based on where the pivot is rather than just the top-left.
+        /// </summary>
+        [NoAnimate]
+        public void Resize(ref Float2 value)
+        {
+            if (_bounds.Size.Equals(ref value))
+                return;
+            var bounds = new Rectangle(_bounds.Location, value);
+            bounds.Location += (_bounds.Size - value) * Pivot; // Pivot-relative resizing
+            SetBounds(ref bounds);
+        }
+
         /// <summary>
         /// Updates the control cached bounds (based on anchors and offsets).
         /// </summary>

Source/Engine/UI/GUI/Panels/DropPanel.cs

@@ -585,7 +585,8 @@ namespace FlaxEngine.GUI
             _cachedHeight = height;
             if (_animationProgress >= 1.0f && _isClosed)
                 y = minHeight;
-            Height = Mathf.Max(minHeight, y);
+            var size = new Float2(Width, Mathf.Max(minHeight, y));
+            Resize(ref size);
         }
 
         /// <inheritdoc />

Source/Engine/UI/GUI/Panels/HorizontalPanel.cs

@@ -78,7 +78,7 @@ namespace FlaxEngine.GUI
                 size.X = left + right;
                 if (!ControlChildSize)
                     size.Y = maxHeight;
-                Size = size;
+                Resize(ref size);
             }
             else if (_alignment != TextAlignment.Near && hasAnyLeft)
             {

Source/Engine/UI/GUI/Panels/VerticalPanel.cs

@@ -78,7 +78,7 @@ namespace FlaxEngine.GUI
                 size.Y = top + bottom;
                 if (!ControlChildSize)
                     size.X = maxWidth;
-                Size = size;
+                Resize(ref size);
             }
             else if (_alignment != TextAlignment.Near && hasAnyTop)
             {