Timeline render bar (green/yellow/red) not rendering anymore

[defaultbceceb9ejb9z]

Something happened today while I was editing, I use to be able to quickly scrub my clips on my timeline WITHOUT having to go to Sequence>Render In and Out. 

Why doesn't the yellow bar automatically render? I could have sworm it use to, I don't know what I clicked or what I did. How can I fix this? is there a setting to set up automatic rendering on the timeline?

[RobShultz]

The yellow bar doesn't render. The yellow bar means that that part of your timeline is rendered in the gpu.

The red line is what you can render by pressing return.