old_contents: var target = document.querySelector('body');
  var config = {
    childList: true,
    subtree: true
  };
  
  var observer = new MutationObserver(function(mutations, self) {
    mutations.forEach(function(mutation) {
      if (mutation.type === 'childList') {
        var elements = document.querySelectorAll('#js-pull-merging .primary.merge-branch-action');
        Array.prototype.forEach.call(elements, function(element) {
          if (document.getElementsByClassName("js-issue-title")[0].textContent > -1) {
            if (element.className.indexOf("primary") > -1) {
              element.className = element.className.replace(/primary/g, 'disabled').replace(/js-details-target/g, '');
            }
          }
        });
      }
    });
  });
  
  observer.observe(target, config);
  
new_contents: var target = document.querySelector('body');
  var config = {
    childList: true,
    subtree: true
  };
  
  var observer = new MutationObserver(function(mutations, self) {
    mutations.forEach(function(mutation) {
      if (mutation.type === 'childList') {
        var elements = document.querySelectorAll('#js-pull-merging .merge-branch-action');
        Array.prototype.forEach.call(elements, function(element) {
          if (document.getElementsByClassName("js-issue-title")[0].textContent.indexOf("WIP") > -1) {
            if (element.className.indexOf("primary") > -1) {
              element.className = element.className.replace(/primary/g, 'disabled').replace(/js-details-target/g, '');
            }
          } else {
            if (element.className.indexOf("disabled") > -1) {
              element.className = element.className.replace(/disabled/g, 'primary js-details-target');
            }
          }
        });
      }
    });
  });
  
  observer.observe(target, config);
  
subject: support to trace changing PR's title