Add tooltip on every buttons such as for eraser, pencils, and others. Also, please fix the UNDO function. It doesn't work properly

Sometimes the undo function cannot function, for example when I incorrectly mark a region, it cannot undo previous mark. Only undo works when I clicked PAINT, and undo, it works just fine. So before I clicked PAINT, if I incorrectly mark a region, sometimes, it cannot be undo. Thanks :)