I see now. The tab name is shown in the title bar of the Arduino IDE. The Arduino IDE doesn't do a great job of making it easy to work with sketches that have lots of tabs. The upcoming version of Marlin is actually going to have only 3 tabs, which are configuration files where the average user might need to edit. The rest of the Marlin source code is in a subfolder where it will not appear in the Arduino IDE. So you would probably end up using a more capable text editor if you needed to make changes to any of those files.