Question

In firefox, I get a delay and it makes scrolling the window (with trackpad on mac) virtually impossible to do it smoothly.

http://wishd.me/

Not sure what I’m doing wrong, if anything, or is this just an unintended side-effect of doing a transition on a hover.

.list > li > div {
  border: 1px solid #09c;
  border-radius: 5px;
  margin-bottom: 20px;
  padding: 10px 20px;
  border-bottom: 7px solid #39c;
  position: relative;
  background: #ffffff;
  /* Old browsers */

  background: -moz-linear-gradient(top, #ffffff 0%, #fcfcfc 66%, #fbfbfb 80%, #fafafa 100%);
  /* FF3.6+ */

  background: -webkit-gradient(linear, left top, left bottom, color-stop(0%, #ffffff), color-stop(66%, #fcfcfc), color-stop(80%, #fbfbfb), color-stop(100%, #fafafa));
  /* Chrome,Safari4+ */

  background: -webkit-linear-gradient(top, #ffffff 0%, #fcfcfc 66%, #fbfbfb 80%, #fafafa 100%);
  /* Chrome10+,Safari5.1+ */

  background: -o-linear-gradient(top, #ffffff 0%, #fcfcfc 66%, #fbfbfb 80%, #fafafa 100%);
  /* Opera 11.10+ */

  background: -ms-linear-gradient(top, #ffffff 0%, #fcfcfc 66%, #fbfbfb 80%, #fafafa 100%);
  /* IE10+ */

  background: linear-gradient(to bottom, #ffffff 0%, #fcfcfc 66%, #fbfbfb 80%, #fafafa 100%);
  /* W3C */

  filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#ffffff', endColorstr='#fafafa', GradientType=0);
  /* IE6-9 */

  /*box-shadow: 1px 5px 15px #f0f0f0; */

  box-shadow: 1px 1px 2px #fff inset, 1px 5px 15px #f0f0f0;
  -webkit-transition: all 0.15s ease-in-out;
  -moz-transition: all 0.15s ease-in-out;
  -o-transition: all 0.15s ease-in-out;
  -ms-transition: all 0.15s ease-in-out;
  transition: all 0.15s ease-in-out;
  margin: 0 10px 20px;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
}
.list > li:hover > div {
  box-shadow: 1px 1px 2px #fff inset, 0px 5px 20px #ddd;
  background: #fff;
  -webkit-transition: all 0.15s ease-in-out;
  -moz-transition: all 0.15s ease-in-out;
  -o-transition: all 0.15s ease-in-out;
  -ms-transition: all 0.15s ease-in-out;
  transition: all 0.15s ease-in-out;
  margin-top: -1px;
  margin-bottom: 21px;
}

Answer 1

I can’t do a through test on your issue but…

Transition styling needs not copy to the :hover section. Only the original section is enough. It could be the culprit.

Since the rendering speed is probably the major source of problem, you may also try adding a transition-delay, so that scrolling won’t activate all the hoverable elements beneath.