Fix `GeometryBuilder.DrawRoundedCornersRectangle` for `Rect` (#21482)

* Use 0 degree rotation angle in DrawRoundedCornersRectangle() for Rect

* Fix units in rotationAngle comment

* Improve ArcTo() XML docs

* Remove unused PiOver2 const causing build checks to fail

---------

Co-authored-by: Steven Kirk <grokys@users.noreply.github.com>

src/Avalonia.Base/Media/GeometryBuilder.cs

@@ -20,7 +20,6 @@ namespace Avalonia.Media
     /// </summary>
     internal class GeometryBuilder
     {
-        private const double PiOver2 = 1.57079633; // 90 deg to rad
         private const double Epsilon = 0.00000153; // Same as LayoutHelper.LayoutEpsilon
 
         /// <summary>
@@ -157,7 +156,7 @@ namespace Avalonia.Media
             context.ArcTo(
                 new Point(rect.Right, rect.Top + radiusY),
                 arcSize,
-                rotationAngle: PiOver2,
+                rotationAngle: 0.0,
                 isLargeArc: false,
                 SweepDirection.Clockwise);
 
@@ -166,7 +165,7 @@ namespace Avalonia.Media
             context.ArcTo(
                 new Point(rect.Right - radiusX, rect.Bottom),
                 arcSize,
-                rotationAngle: PiOver2,
+                rotationAngle: 0.0,
                 isLargeArc: false,
                 SweepDirection.Clockwise);
 
@@ -175,7 +174,7 @@ namespace Avalonia.Media
             context.ArcTo(
                 new Point(rect.Left, rect.Bottom - radiusY),
                 arcSize,
-                rotationAngle: PiOver2,
+                rotationAngle: 0.0,
                 isLargeArc: false,
                 SweepDirection.Clockwise);
 
@@ -184,7 +183,7 @@ namespace Avalonia.Media
             context.ArcTo(
                 new Point(rect.Left + radiusX, rect.Top),
                 arcSize,
-                rotationAngle: PiOver2,
+                rotationAngle: 0.0,
                 isLargeArc: false,
                 SweepDirection.Clockwise);
 

src/Avalonia.Base/Platform/IGeometryContext.cs

@@ -11,14 +11,19 @@ namespace Avalonia.Platform
         /// <summary>
         /// Draws an arc to the specified point.
         /// </summary>
-        /// <param name="point">The destination point.</param>
-        /// <param name="size">The radii of an oval whose perimeter is used to draw the angle.</param>
-        /// <param name="rotationAngle">The rotation angle (in radians) of the oval that specifies the curve.</param>
-        /// <param name="isLargeArc">true to draw the arc greater than 180 degrees; otherwise, false.</param>
+        /// <param name="point">The destination point where the arc ends.</param>
+        /// <param name="size">The radii of an ellipse whose path is used to draw the arc.</param>
+        /// <param name="rotationAngle">
+        ///   The rotation angle (in degrees) of the ellipse that specifies the path of the arc; positive values are clockwise.
+        ///   This will rotate the entire arc relative to the X-axis and is not commonly used.
+        /// </param>
+        /// <param name="isLargeArc">
+        ///   Specifies whether the arc should follow the longer path around an ellipse rather than the shorter one.
+        ///   <c>true</c> to draw the arc greater than 180 degrees; otherwise, <c>false</c>.</param>
         /// <param name="sweepDirection">
         /// A value that indicates whether the arc is drawn in the Clockwise or Counterclockwise direction.
         /// </param>
-        /// <param name="isStroked">Whether the segment is stroked</param>
+        /// <param name="isStroked">Whether the segment is stroked.</param>
         void ArcTo(Point point, Size size, double rotationAngle, bool isLargeArc, SweepDirection sweepDirection, bool isStroked = true);
 
         /// <summary>
@@ -34,22 +39,22 @@ namespace Avalonia.Platform
         /// <param name="controlPoint1">The first control point used to specify the shape of the curve.</param>
         /// <param name="controlPoint2">The second control point used to specify the shape of the curve.</param>
         /// <param name="endPoint">The destination point for the end of the curve.</param>
-        /// <param name="isStroked">Whether the segment is stroked</param>
+        /// <param name="isStroked">Whether the segment is stroked.</param>
         void CubicBezierTo(Point controlPoint1, Point controlPoint2, Point endPoint, bool isStroked = true);
 
         /// <summary>
-        /// Draws a quadratic Bezier curve to the specified point
+        /// Draws a quadratic Bezier curve to the specified point.
         /// </summary>
         /// <param name="controlPoint">The control point used to specify the shape of the curve.</param>
         /// <param name="endPoint">The destination point for the end of the curve.</param>
-        /// <param name="isStroked">Whether the segment is stroked</param>
+        /// <param name="isStroked">Whether the segment is stroked.</param>
         void QuadraticBezierTo(Point controlPoint, Point endPoint, bool isStroked = true);
 
         /// <summary>
         /// Draws a line to the specified point.
         /// </summary>
         /// <param name="point">The destination point.</param>
-        /// <param name="isStroked">Whether the segment is stroked</param>
+        /// <param name="isStroked">Whether the segment is stroked.</param>
         void LineTo(Point point, bool isStroked = true);
 
         /// <summary>