Fix flex overflow render in Safari

[trap000d]

There are some people still using "vintage" Apple. Proposed PR fixes playlist scrollbars render in HighSierra/Safari. To be more precise, currently MStream CSS contains some bug in flexbox containers hierarchy (google: "flex overflow", e.g. https://stackoverflow.com/questions/43502451/nested-flexbox-ui-with-scrolling-content-rendered-differently-in-safari-and-in-m), which was pretty annoying several years ago, but by now managed to be handled by modern browsers.