Fix AllocateBackBuffer to only add padding for sub-pixel offsets

Expand the back buffer by +1 pixel only when a fractional screen offset
would cause content to overflow, rather than always adding padding.
This preserves correct border rendering at integer positions.

Made-with: Cursor

Author: larsbrubaker

Gui/GUIWidget.cs

@@ -814,11 +814,15 @@ public virtual void OnContainsFocusChanged(FocusChangedArgs e)
 		}
 
 		private void AllocateBackBuffer()
+		{
+			AllocateBackBuffer(0, 0);
+		}
+
+		private void AllocateBackBuffer(int extraWidth, int extraHeight)
 		{
 			RectangleDouble localBounds = LocalBounds;
-			// +1 accounts for sub-pixel offsets when compositing at fractional screen positions
-			int intWidth = Max((int)(Ceiling(localBounds.Right) - Floor(localBounds.Left)) + 1, 1);
-			int intHeight = Max((int)(Ceiling(localBounds.Top) - Floor(localBounds.Bottom)) + 1, 1);
+			int intWidth = Max((int)(Ceiling(localBounds.Right) - Floor(localBounds.Left)) + extraWidth, 1);
+			int intHeight = Max((int)(Ceiling(localBounds.Top) - Floor(localBounds.Bottom)) + extraHeight, 1);
 			if (backBuffer == null || backBuffer.Width != intWidth || backBuffer.Height != intHeight)
 			{
 				backBuffer = new ImageBuffer(intWidth, intHeight, 32, new BlenderPreMultBGRA());
@@ -2130,6 +2134,13 @@ public virtual void OnDraw(Graphics2D graphics2D)
 								int yOffset = (int)Floor(child.LocalBounds.Bottom);
 								if (child.isCurrentlyInvalid)
 								{
+									int extraW = xFraction > 0 ? 1 : 0;
+									int extraH = yFraction > 0 ? 1 : 0;
+									if (extraW > 0 || extraH > 0)
+									{
+										child.AllocateBackBuffer(extraW, extraH);
+									}
+
 									Graphics2D childBackBufferGraphics2D = child.backBuffer.NewGraphics2D();
 									childBackBufferGraphics2D.Clear(new Color(0, 0, 0, 0));
 									var transformToBuffer = Affine.NewTranslation(-xOffset + xFraction, -yOffset + yFraction);