[js] IE yellow download bar

Say you have a "download" link. Clicking it kicks off a Javascript function that ultimately navigates the browser to your download PHP script, which serves up the file via something like:

/* download.php */
..
header('Cache-Control: public'); 
header("Content-type: application/pdf");
header('Content-Disposition: attachment; filename="' . $filename . '"');
readfile($path . $filename);

In IE, this usually pops up a "File Download" dialog asking if you want to open or save the file; pretty convenient. But something went awry somewhere. You're now getting a yellow bar warning instead. Why?

To help protect your security, Internet Explorer blocked this site from downloading files to your computer. Click here for options...

Check your Javascript. IE will allow you, the developer, to initiate a download from a click event as long as you do so within the same execution thread. Let the click thread end, though, and try to navigate to the download script from another thread, and, hello, yellow bar. In other words, look out for setTimeout.

  download_onclick:function(id) {
    window.location.href = 'download.php?id=' + id;    // good to go
  }, ..

  download_onclick:function(id) {
    setTimeout(function() {
      window.location.href = 'download.php?id=' + id;  // yellow bar
    }, 1);
  }, ..