File tree browser post rollout small styling fixes

This issue groups a few small styling issues found Test session: File Tree Browser (#582189). All should be quick win, if not, we should open separate issues for them.

• Rounding corners with broadcast message #582189 (comment 2927654802)
• Non-default text sizes causes the loading icon to move the text #582189 (comment 2927694544)
• Selected state highlight does not accomodate horizontal overflow #582189 (comment 2929178692)
• Show more hover state is visually odd #582189 (comment 2929217663)
• Overlapping hover state for directory line and caret icon #582189 (comment 2931695014)
• Caret isn't aligned with the vertical line for open directories #582189 (comment 2931704851)
• Scroll bar obscures file tree browser content [nitpick] #582189 (comment 2931720224)