Yes, this is an existing problem.
I don't believe OnlineGDB was designed to handle many files.
You can reduce the impact by
- maximizing your window
- closing any unnecessary side panels
- changing the zoom in your browser temporarily so you can click the tab you want, then change it back (<Ctrl>+<mouse scroll> can do it rather quickly).
Perhaps the owners will address this issue and make the file list scrollable in the future.