Auto merge of #17278 - asajeffrey:canvas-clear-webrender-image-key-when-resizing, r=emilio

Clear webrender image id when resizing a canvas.

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Webrender isn't very happy if images change size, so clear the webrender image key when resizing a canvas.

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #17277
- [X] There are tests for these changes

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components/canvas/canvas_paint_thread.rs

@@ -547,6 +547,10 @@ impl<'a> CanvasPaintThread<'a> {
 
     fn recreate(&mut self, size: Size2D<i32>) {
         self.drawtarget = CanvasPaintThread::create(size);
+        // Webrender doesn't let images change size, so we clear the webrender image key.
+        if let Some(image_key) = self.image_key.take() {
+            self.webrender_api.delete_image(image_key);
+        }
     }
 
     fn send_pixels(&mut self, chan: IpcSender<Option<Vec<u8>>>) {

components/canvas/webgl_paint_thread.rs

@@ -291,7 +291,7 @@ impl WebGLPaintThread {
     #[allow(unsafe_code)]
     fn recreate(&mut self, size: Size2D<i32>) -> Result<(), &'static str> {
         match self.data {
-            WebGLPaintTaskData::Readback(ref mut context, _, _) => {
+            WebGLPaintTaskData::Readback(ref mut context, ref webrender_api, ref mut image_key) => {
                 if size.width > self.size.width ||
                    size.height > self.size.height {
                     self.size = try!(context.resize(size));
@@ -299,6 +299,10 @@ impl WebGLPaintThread {
                     self.size = size;
                     context.gl().scissor(0, 0, size.width, size.height);
                 }
+                // Webrender doesn't let images change size, so we clear the webrender image key.
+                if let Some(image_key) = image_key.take() {
+                    webrender_api.delete_image(image_key);
+                }
             }
             WebGLPaintTaskData::WebRender(ref api, id) => {
                 let device_size = webrender_traits::DeviceIntSize::from_untyped(&size);