optimizing relseasing canvas

web/pdf_thumbnail_view.js

@@ -182,9 +182,9 @@ var PDFThumbnailView = (function PDFThumbnailViewClosure() {
       var canvas = document.createElement('canvas');
       //canvas.id = this.renderingId;
 
-      canvas.className = 'thumbnailImage';
-      canvas.setAttribute('aria-label', mozL10n.get('thumb_page_canvas',
-        {page: this.id}, 'Thumbnail of Page {{page}}'));
+      //canvas.className = 'thumbnailImage';
+      //canvas.setAttribute('aria-label', mozL10n.get('thumb_page_canvas',
+      //  {page: this.id}, 'Thumbnail of Page {{page}}'));
 
       this.canvas = canvas;
       this.div.setAttribute('data-loaded', true);
@@ -203,6 +203,9 @@ var PDFThumbnailView = (function PDFThumbnailViewClosure() {
       this.image.id = this.renderingId;
       this.image.style.height = canvas.style.height;
       this.image.style.width = canvas.style.width;
+      this.image.className = 'thumbnailImage';
+      this.image.setAttribute('aria-label', mozL10n.get('thumb_page_canvas',
+            {page: this.id}, 'Thumbnail of Page {{page}}'));
       this.ring.appendChild(this.image);
       return ctx;
     },
@@ -318,7 +321,9 @@ var PDFThumbnailView = (function PDFThumbnailViewClosure() {
       ctx.drawImage(reducedImage, 0, 0, reducedWidth, reducedHeight,
                     0, 0, canvas.width, canvas.height);
       this.image.src = canvas.toDataURL();
-      ctx.clearRect(0, 0, canvas.style.width, canvas.style.height);
+      this.canvas.width = 0;
+      this.canvas.height = 0;
+      delete this.canvas;
     }
   };