servo: Merge #7161 - Snap rectangles to nearest pixels consistently (from mbrubeck:underline); r=pcwalton

Snapping the top-left and bottom-right corners separately can cause a rectangle to change size or even become empty when offset by a subpixel amount.  Instead, this patch snaps the top-left corner, then snaps the size to a whole pixel amount, so any rectangle of a given original size will always have the same snapped size.

Fixes #7152. r? @pcwalton or @glennw

Source-Repo: https://github.com/servo/servo
Source-Revision: 9c528c6382ee4355f42824a7caad217c4eb8a3b0

servo/components/gfx/paint_context.rs

@@ -1171,16 +1171,10 @@ pub trait ToAzureRect {
 
 impl ToAzureRect for Rect<Au> {
     fn to_nearest_azure_rect(&self) -> Rect<AzFloat> {
-        let top_left = self.origin.to_nearest_azure_point();
-        let bottom_right = self.bottom_right().to_nearest_azure_point();
-        Rect::new(top_left,
-                  Size2D::new((bottom_right.x - top_left.x) as AzFloat,
-                              (bottom_right.y - top_left.y) as AzFloat))
-
+        Rect::new(self.origin.to_nearest_azure_point(), self.size.to_nearest_azure_size())
     }
     fn to_azure_rect(&self) -> Rect<AzFloat> {
-        Rect::new(self.origin.to_azure_point(), Size2D::new(self.size.width.to_f32_px(),
-                                                            self.size.height.to_f32_px()))
+        Rect::new(self.origin.to_azure_point(), self.size.to_azure_size())
     }
 }
 
@@ -1382,11 +1376,12 @@ impl DrawTargetExtensions for DrawTarget {
     }
 
     fn create_rectangular_path(&self, rect: &Rect<Au>) -> Path {
+        let rect = rect.to_nearest_azure_rect();
         let path_builder = self.create_path_builder();
-        path_builder.move_to(rect.origin.to_nearest_azure_point());
-        path_builder.line_to(Point2D::new(rect.max_x(), rect.origin.y).to_nearest_azure_point());
-        path_builder.line_to(Point2D::new(rect.max_x(), rect.max_y()).to_nearest_azure_point());
-        path_builder.line_to(Point2D::new(rect.origin.x, rect.max_y()).to_nearest_azure_point());
+        path_builder.move_to(rect.origin);
+        path_builder.line_to(Point2D::new(rect.max_x(), rect.origin.y));
+        path_builder.line_to(Point2D::new(rect.max_x(), rect.max_y()));
+        path_builder.line_to(Point2D::new(rect.origin.x, rect.max_y()));
         path_builder.finish()
     }
 }