window.open must be called within a callback which is triggered by a user action (example onclick) for the page to open in a new tab instead of a window.
Example:
$("a.runReport").click(function(evt) {
// open a popup within the click handler
// this should open in a new tab
var popup = window.open("about:blank", "myPopup");
//do some ajax calls
$.get("/run/the/report", function(result) {
// now write to the popup
popup.document.write(result.page_content);
// or change the location
// popup.location = 'someOtherPage.html';
});
});