What means dashed chart in Chrome DevTool Timeline

Question

I have low FPS when start scrolling. I use DevTool Timeline to find out the reasons for this. But I can't find what means dashed chart. Help me please!

Answer 1

It means the work is done by a background thread, rather than by the main thread.

Take this screenshot as an example. A small amount of code runs on the main thread, followed by most of the rendering happening on other threads.