doc: add more information about webgl problems

README.md

@@ -33,6 +33,13 @@ npm run dev
 
 ### Known issues
 
+#### Canvas
+When `preserveDrawingBuffer` is set to false, we save 1x1 empty image because problems with access to drawing buffer.
+see [https://stackoverflow.com/a/27747016](https://stackoverflow.com/a/27747016)
+```
+can.getContext("webgl2", { preserveDrawingBuffer: true })
+```
+
 #### Shadow root  
 Open shadow root is not always displayed or parsed correctly - but at least it matches mht quality or sometimes it's better.  
 Saving fragment / adding pin to shadow root is not working right now.

src/content-script/factory/html/html.factory.ts

@@ -68,7 +68,9 @@ export class HtmlFactory {
           const gl = render.ctx as WebGLRenderingContext;
           if (gl.getContextAttributes()?.preserveDrawingBuffer) {
             imgData = ref.toDataURL('image/png', 80);
+            fnConsoleLog('HtmlFactory->computeCanvas->preserveDrawingBuffer', true);
           } else {
+            fnConsoleLog('HtmlFactory->computeCanvas->preserveDrawingBuffer', false);
             /* TODO capture webgl texture without preserveDrawingBuffer
             const texture = gl.createTexture();
             gl.bindTexture(gl.TEXTURE_2D, texture);

src/service-worker/command/sync/update/sync-gather-changes.command.ts

@@ -64,6 +64,10 @@ export class SyncGatherChangesCommand implements ICommand<Promise<ServerChangeDt
 
     const comments = this.commentChanges(pageObj.comments.data);
     changes.push(...comments);
+    if (!pageObj.draw.data) {
+      pageObj.draw = { data: [] };
+      await BrowserStorageWrapper.set(`${ObjectStoreKeys.OBJECT_ID}:${this.obj.id}`, this.obj);
+    }
     const draw = this.drawChanges(pageObj.draw.data);
     changes.push(...draw);
     return changes;