The way CSS is implemented in classic editor makes it difficult to cleanly alter or override the container’s default styling. In any case, I suspect a script issue, not a CSS issue.
It’s normal for a scroll bar to briefly appear before the editor has fully loaded. It is supposed to be removed and the editor container resized to contain all of the content. As you add/remove content, the container continually gets resized to accommodate the changes. If the container is not resized and some of your content is hidden, then there’s likely a script error preventing the auto-resizing to work correctly. The error is likely due to something your theme or a plugin is doing.
Narrow down the cause by switching to a default Twenty* theme and deactivating all plugins except for Classic Editor. You should now see all of your content and no scroll bar for the editor container. The entire window may have a scrollbar, but not the editor container itself. Restore your normal theme and plugins, one at a time, until the problem recurs. The last activated module would be the cause. Take up the issue through that module’s dedicated support channel.
