Our floobits range highlighter has been freezing the IntelliJ GUI when a lot of text was selected and I finally traced it down to the alpha value.
I created a project here to demo the problem:
Run intellij with plugin and then select the action it adds "highlight all text" in a decently sized file and then try to scroll and notice how the scroll is locked up.
Then change the value of this line:
|attributes.setBackgroundColor(new Color(255, 0, 0, 50));|
and try again and see the UI does not freeze up.