fix: Make sure that an element can invalidate itself (and its parents…

…) while in being measured / arranged

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml/Given_FrameworkElement.cs

@@ -160,6 +160,47 @@ public Task When_Measure_And_Invalidate() =>
 				Assert.AreEqual(new Size(10, 10), SUT.MeasureOverrides[1]);
 			});
 
+
+		[TestMethod]
+		[RunsOnUIThread]
+		public async Task When_InvalidateDuringMeasure_Then_GetReMeasured()
+		{
+			var sut = new ObservableLayoutingControl();
+			var count = 0;
+			sut.OnMeasure += (snd, e) =>
+			{
+				if (count++ == 0)
+				{
+					snd.InvalidateMeasure();
+				}
+			};
+
+			TestServices.WindowHelper.WindowContent = sut;
+			await TestServices.WindowHelper.WaitForIdle();
+
+			Assert.AreEqual(2, count);
+		}
+
+		[TestMethod]
+		[RunsOnUIThread]
+		public async Task When_InvalidateDuringArrange_Then_GetReArranged()
+		{
+			var sut = new ObservableLayoutingControl();
+			var count = 0;
+			sut.OnArrange += (snd, e) =>
+			{
+				if (count++ == 0)
+				{
+					snd.InvalidateArrange();
+				}
+			};
+
+			TestServices.WindowHelper.WindowContent = sut;
+			await TestServices.WindowHelper.WaitForIdle();
+
+			Assert.AreEqual(2, count);
+		}
+
 		[TestMethod]
 		public Task MeasureWithNan() =>
 			RunOnUIThread.ExecuteAsync(() =>
@@ -649,4 +690,24 @@ protected override Size MeasureOverride(Size availableSize)
 			return new Size(width, height);
 		}
 	}
+
+	public partial class ObservableLayoutingControl : FrameworkElement
+	{
+		public event TypedEventHandler<ObservableLayoutingControl, Size> OnMeasure;
+
+		public event TypedEventHandler<ObservableLayoutingControl, Size> OnArrange;
+
+		protected override Size MeasureOverride(Size availableSize)
+		{
+			OnMeasure?.Invoke(this, availableSize);
+			return new Size(100, 100);
+		}
+
+		/// <inheritdoc />
+		protected override Size ArrangeOverride(Size finalSize)
+		{
+			OnArrange?.Invoke(this, finalSize);
+			return new Size(100, 100);
+		}
+	}
 }

src/Uno.UI/UI/Xaml/UIElement.Layout.netstd.cs

@@ -133,9 +133,11 @@ private void DoMeasure(Size availableSize)
 		{
 			InvalidateArrange();
 
+			// We must reset the flag **BEFORE** doing the actual measure, so the elements are able to re-invalidate themselves
+			_isMeasureValid = true;
+
 			MeasureCore(availableSize);
 			LayoutInformation.SetAvailableSize(this, availableSize);
-			_isMeasureValid = true;
 		}
 
 		internal virtual void MeasureCore(Size availableSize)
@@ -205,8 +207,10 @@ private void DoArrange(Rect finalRect)
 			// For instance, the EffectiveViewPort computation reads that value to detect slot changes (cf. PropagateEffectiveViewportChange)
 			LayoutInformation.SetLayoutSlot(this, finalRect);
 
-			ArrangeCore(finalRect);
+			// We must reset the flag **BEFORE** doing the actual arrange, so the elements are able to re-invalidate themselves
 			_isArrangeValid = true;
+
+			ArrangeCore(finalRect);
 		}
 
 		partial void HideVisual();