mergeMap((contentRef: ContentRef) => {
      const state = state$.value;
      const content = selectors.content(state, { contentRef });

      let isVisible = false;

      // document.hidden appears well supported
      if (document.hidden) {
        // Opera 12.10 and Firefox 18 and later support
        isVisible = !document.hidden;
      } else if ((document as any).msHidden) {
        isVisible = !(document as any).msHidden;
      } else if ((document as any).webkitHidden) {
        isVisible = !(document as any).webkitHidden;
      } else {
        // Final fallback -- this will say the window is hidden when devtools is open or if the
        // user is interacting with an iframe
        isVisible = document.hasFocus();
      }

      if (
        isVisible &&
        // Don't bother saving nothing
        content &&
        // Only files and notebooks
        (content.type === "file" || content.type === "notebook") &&
        // Only save if they have a real filepath
        content.filepath !== ""
      ) {
        return of(actions.save({ contentRef }));
      } else {
        return empty();
      }
    })

 test("should set isSaving to true", () => {
   const originalState = stateModule.makeAppRecord({
     isSaving: false,
     kernel: stateModule.makeLocalKernelRecord({
       channels: false,
       spawn: false,
       connectionFile: false
     })
   });
   const state = reducers.app(originalState, actions.save({}));
   expect(state.isSaving).toBe(true);
 });