In both the Viewer and Compositor windows, there is an arrow on the right hand side of the other buttons in the edit panel as shown in figure 2.18. The "play" button can be toggled on/off via this arrow, which has a tooltip of Click to play . When enabled the arrow is white surrounded by green and when disabled the arrow is red.4The purpose of enabling this capability is to make it really easy to play the media in the window by just using the left mouse button to start or stop the play. The entire main canvas surface becomes a big play button! Although the default is initially off, a good reason to enable this, at least temporarily, is so that you can quickly review your video before a render.