fix(scrollbar): Fix possible layout cycle when scrollbar is lower than the min size of the thumb

Author: dr1rrb

src/Uno.UI/UI/Xaml/Controls/ScrollBar/ScrollBar.cs

@@ -1146,6 +1146,13 @@ void UpdateIndicatorLengths(
 			pMouseIndicatorLength = 0.0;
 			pTouchIndicatorLength = 0.0;
 
+			// Uno workaround: If the scrollbar is smaller than the min size of the thumb,
+			//		we will try to set the visibility collapsed and then request to 'HideThumb',
+			//		which will drive uno to fall in an infinite layout cycle.
+			//		We instead cache the value and apply it only at the end.
+			double? m_tpElementHorizontalThumbWidth = default, m_tpElementVerticalThumbHeight = default, m_tpElementHorizontalPanningThumbWidth = default, m_tpElementVerticalPanningThumbHeight = default;
+			Visibility? m_tpElementHorizontalThumbVisibility = default, m_tpElementVerticalThumbVisibility = default, m_tpElementHorizontalPanningThumbVisibility = default, m_tpElementVerticalPanningThumbVisibility = default;
+
 			if (!hideThumb)
 			{
 				double minSize = 0.0;
@@ -1159,8 +1166,9 @@ void UpdateIndicatorLengths(
 
 				double actualSize;
 				double actualSizeMinusRepeatButtonsLength;
+
 				if (orientation == Orientation.Horizontal &&
-m_tpElementHorizontalThumb != null)
+					m_tpElementHorizontalThumb != null)
 				{
 					if (maximum - minimum != 0)
 					{
@@ -1177,8 +1185,8 @@ void UpdateIndicatorLengths(
 					}
 					else
 					{
-						m_tpElementHorizontalThumb.Visibility = Visibility.Visible;
-						m_tpElementHorizontalThumb.Width = result;
+						m_tpElementHorizontalThumbVisibility = Visibility.Visible;
+						m_tpElementHorizontalThumbWidth = result;
 						mouseIndicatorLengthWasSet = true;
 					}
 				}
@@ -1200,8 +1208,8 @@ void UpdateIndicatorLengths(
 					}
 					else
 					{
-						m_tpElementVerticalThumb.Visibility = Visibility.Visible;
-						m_tpElementVerticalThumb.Height = result;
+						m_tpElementVerticalThumbVisibility = Visibility.Visible;
+						m_tpElementVerticalThumbHeight = result;
 						mouseIndicatorLengthWasSet = true;
 					}
 				}
@@ -1233,8 +1241,8 @@ void UpdateIndicatorLengths(
 					}
 					else
 					{
-						m_tpElementHorizontalPanningThumb.Visibility = Visibility.Visible;
-						m_tpElementHorizontalPanningThumb.Width = result;
+						m_tpElementHorizontalPanningThumbVisibility = Visibility.Visible;
+						m_tpElementHorizontalPanningThumbWidth = result;
 						touchIndicatorLengthWasSet = true;
 					}
 				}
@@ -1255,8 +1263,8 @@ void UpdateIndicatorLengths(
 					}
 					else
 					{
-						m_tpElementVerticalPanningThumb.Visibility = Visibility.Visible;
-						m_tpElementVerticalPanningThumb.Height = result;
+						m_tpElementVerticalPanningThumbVisibility = Visibility.Visible;
+						m_tpElementVerticalPanningThumbHeight = result;
 						touchIndicatorLengthWasSet = true;
 					}
 				}
@@ -1287,6 +1295,17 @@ void UpdateIndicatorLengths(
 					m_tpElementVerticalPanningThumb.Visibility = Visibility.Collapsed;
 				}
 			}
+			else
+			{
+				if (m_tpElementHorizontalThumbWidth.HasValue) m_tpElementHorizontalThumb.Width = m_tpElementHorizontalThumbWidth.Value;
+				if (m_tpElementVerticalThumbHeight.HasValue) m_tpElementVerticalThumb.Height = m_tpElementVerticalThumbHeight.Value;
+				if (m_tpElementHorizontalPanningThumbWidth.HasValue) m_tpElementHorizontalPanningThumb.Width = m_tpElementHorizontalPanningThumbWidth.Value;
+				if (m_tpElementVerticalPanningThumbHeight.HasValue) m_tpElementVerticalPanningThumb.Height = m_tpElementVerticalPanningThumbHeight.Value;
+				if (m_tpElementHorizontalThumbVisibility.HasValue) m_tpElementHorizontalThumb.Visibility = m_tpElementHorizontalThumbVisibility.Value;
+				if (m_tpElementVerticalThumbVisibility.HasValue) m_tpElementVerticalThumb.Visibility = m_tpElementVerticalThumbVisibility.Value;
+				if (m_tpElementHorizontalPanningThumbVisibility.HasValue) m_tpElementHorizontalPanningThumb.Visibility = m_tpElementHorizontalPanningThumbVisibility.Value;
+				if (m_tpElementVerticalPanningThumbVisibility.HasValue) m_tpElementVerticalPanningThumb.Visibility = m_tpElementVerticalPanningThumbVisibility.Value;
+			}
 		}
 
 		// during a SemanticZoomOperation we want to be able to block the scrollbar