Account for RenderLayer position when optimizing display list

The page_rect passed to DisplayListOptimizer is relative to the
RenderLayer origin, but the display list components are relative to the
page origin. Before passing the page rect to the display list, we
translate it by the RenderLayer position.

1 files changed, 11 insertions, 5 deletions

diff --git a/components/gfx/render_task.rs b/components/gfx/render_task.rs
index e19db054239..92d9a2de2b0 100644
--- a/components/gfx/render_task.rs
+++ b/components/gfx/render_task.rs
@@ -13,6 +13,7 @@ use render_context::RenderContext;
 use azure::azure_hl::{B8G8R8A8, Color, DrawTarget, StolenGLResources};
 use azure::AzFloat;
 use geom::matrix2d::Matrix2D;
+use geom::point::Point2D;
 use geom::rect::Rect;
 use geom::size::Size2D;
 use layers::platform::surface::{NativePaintingGraphicsContext, NativeSurface};
@@ -299,8 +300,15 @@ impl<C:RenderListener + Send> RenderTask<C> {
 
             // Divide up the layer into tiles.
             for tile in tiles.iter() {
+                // page_rect is in coordinates relative to the layer origin, but all display list
+                // components are relative to the page origin. We make page_rect relative to
+                // the page origin before passing it to the optimizer.
+                let page_rect =
+                     tile.page_rect.translate(&Point2D(render_layer.position.origin.x as f32,
+                                                       render_layer.position.origin.y as f32));
+                let page_rect_au = geometry::f32_rect_to_au_rect(page_rect);
+
                 // Optimize the display list for this tile.
-                let page_rect_au = geometry::f32_rect_to_au_rect(tile.page_rect);
                 let optimizer = DisplayListOptimizer::new(render_layer.display_list.clone(),
                                                           page_rect_au);
                 let display_list = optimizer.optimize();
@@ -339,10 +347,8 @@ impl<C:RenderListener + Send> RenderTask<C> {
                     // Apply the translation to render the tile we want.
                     let matrix: Matrix2D<AzFloat> = Matrix2D::identity();
                     let matrix = matrix.scale(scale as AzFloat, scale as AzFloat);
-                    let matrix = matrix.translate(-(tile.page_rect.origin.x) as AzFloat,
-                                                  -(tile.page_rect.origin.y) as AzFloat);
-                    let matrix = matrix.translate(-(render_layer.position.origin.x as AzFloat),
-                                                  -(render_layer.position.origin.y as AzFloat));
+                    let matrix = matrix.translate(-page_rect.origin.x as AzFloat,
+                                                  -page_rect.origin.y as AzFloat);
 
                     ctx.draw_target.set_transform(&matrix);