A new checkbox has been added to the planetary tax screen entitled "Auto adjust taxes to maintain happiness levels". This checkbox is quite simple, its purpose is to adjust tax rates up or down (usually down) to keep happiness at the level that you have specified with your tax rate.
For example, you set the tax level on your natives so that they will have a 0 happiness adjustment. Then several turns from now your auto build has added more factories and now your tax rate is actually causing -1 happiness. The taxes will be auto adjusted down 1% to maintain the 0 happiness that you are targeting. This is a step toward developing one-touch planets. So that you don't discover 20 turns later that you have very unhappy natives that you didn't intend.
The checkbox is entirely optional and off by default. But you can turn it on if you want to keep your natives at a consistent happiness level.