Do not try to read pixels from an FBO without read buffer

components/script/dom/webgl2renderingcontext.rs

@@ -349,7 +349,11 @@ impl WebGL2RenderingContext {
         }
 
         let fb_slot = self.base.get_draw_framebuffer_slot();
-        if fb_slot.get().is_none() && self.default_fb_readbuffer.get() == constants::NONE {
+        let fb_readbuffer_valid = match fb_slot.get() {
+            Some(fb) => fb.attachment(fb.read_buffer()).is_some(),
+            None => self.default_fb_readbuffer.get() != constants::NONE,
+        };
+        if !fb_readbuffer_valid {
             return self.base.webgl_error(InvalidOperation);
         }
 

tests/wpt/webgl/meta/conformance2/renderbuffers/readbuffer.html.ini

@@ -1,8 +1,2 @@
 [readbuffer.html]
   expected: TIMEOUT
-  [WebGL test #3: gl.getParameter(gl.READ_BUFFER) should be 1029 (of type number). Was null (of type object).]
-    expected: FAIL
-
-  [WebGL test #4: successfullyParsed should be true (of type boolean). Was undefined (of type undefined).]
-    expected: FAIL
-