Q:

unable to expand the event div to entire screen on overflow

https://stackblitz.com/edit/horizontal-tl
please refer the above link before
There is a div with class named 'timelinebar' which is straingline tracked to show on top of list of div with class named 'events' and these div's are overflown on scroll x axis , issue is that timelinebar class div even with width of 100% is not covering all the underlying elements

A:

There is a simple solution, wrap your mainDiv in a container (such as mainDivContainer for example) and set this container to position: relative; instead of mainDiv. The timelinebar will now be relative to the content and not the viewport.
Here is a fork of your stackblitz
And here is an interesting article regarding the subject.

