Earlier today, Microsoft released the first in what is said to be a monthly update for Windows 8.1 called the 'August Update'. While the update is only starting to roll out to users via Windows Update, some individuals are reporting a 'persistent taskbar' at the bottom of modern apps that was not present prior to the update.
Prior to the update, you could access the taskbar by moving your mouse to the bottom of the screen in a gesture like motion to have it pull up for easy app switching. Following the update, the taskbar is staying open for some users but it's not a consistent experience. When using the hot corner to switch apps, the taskbar goes away when using the app. It's an inconsistent experience and so far, the documentation with the update does not hint at this functionality.
If you are seeing the taskbar showing up when using modern apps like the .gif above, let us know in the comments below. We have reached out to Microsoft to see if this is intended functionality or if this is a bug that has made its way into a production release for Windows 8.1 and will report back when we get an answer.
Update: To fix the issue, turn on “Animate windows when minimizing and maximizing” in Performance Options; the issue should go away.