This just started 5 minutes ago. Never seen it before in the 3 years I've owned a Mac. The iMac it's happening on is a 24-incher I bought in 2006. It's only affecting TextEdit right now but I use that app daily and this is becoming very annoying. Everything is highlighted in yellow and I can't turn it off. EVEN the doc title and save buttons. Anyone seen this before or know what to do? Here are some screenshots.