This window is mainly used as an unzoomed preview and to play back an animation. But there is also a zoom slider to zoom in or out.
The title bar contains an icon for additional settings:
Fit Window resizes the window borders so that it fits the image size as it is displayed.
Fit Canvas changes the zoom value so that the canvas fits into the current window.
Center on Playback will center the window to the screen when playing an animation.
Follow Magnify will reposition the focused canvas area to the position where a modification is applied in the magnify window.
Use the speed icons to change animation speed during playback.
< 1:1 allows zoom values smaller that 1:1.