Auto merge of #8728 - dzbarsky:reset_bitmpa, r=dzbarsky

Reset canvas state when changing bitmap dimensions

Ran http://mxr.mozilla.org/servo/source/tests/wpt/web-platform-tests/html/semantics/embedded-content/the-canvas-element/initial.reset.2dstate.html?force=1#88 to confirm that this code is being executed, but the test still fails because the font isn't implemented and the expando doesn't get reset.

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components/script/dom/canvasrenderingcontext2d.rs

@@ -143,12 +143,20 @@ impl CanvasRenderingContext2D {
                            CanvasRenderingContext2DBinding::Wrap)
     }
 
-    pub fn recreate(&self, size: Size2D<i32>) {
+    // https://html.spec.whatwg.org/multipage/#concept-canvas-set-bitmap-dimensions
+    pub fn set_bitmap_dimensions(&self, size: Size2D<i32>) {
+        self.reset_to_initial_state();
         self.ipc_renderer
             .send(CanvasMsg::Common(CanvasCommonMsg::Recreate(size)))
             .unwrap();
     }
 
+    // https://html.spec.whatwg.org/multipage/#reset-the-rendering-context-to-its-default-state
+    fn reset_to_initial_state(&self) {
+        self.saved_states.borrow_mut().clear();
+        *self.state.borrow_mut() = CanvasContextState::new();
+    }
+
     pub fn ipc_renderer(&self) -> IpcSender<CanvasMsg> {
         self.ipc_renderer.clone()
     }

components/script/dom/htmlcanvaselement.rs

@@ -81,7 +81,7 @@ impl HTMLCanvasElement {
         let size = self.get_size();
         if let Some(ref context) = *self.context.borrow() {
             match *context {
-                CanvasContext::Context2d(ref context) => context.recreate(size),
+                CanvasContext::Context2d(ref context) => context.set_bitmap_dimensions(size),
                 CanvasContext::WebGL(ref context) => context.recreate(size),
             }
         }