It's not possible to mark as unsolved.
You didn't mention you were doing this in full screen mode in your first post.
Try troubleshooting extensions and third party plug-ins.
From your Safari menu bar click Safari > Preferences then select the Extensions tab. Turn that off if there are any installed. Quit and relaunch Safari to test. If that helped, turn extensions back on then uninstall one a time to test.
If it's not an extensions issue, try troubleshooting third party plug-ins.
Back to Safari > Preferences. This time select the Security tab. Deselect: Allow all other plug-ins. Quit and relaunch Safari to test.
If that made a difference, instructions for troubleshooting plugins here.
If nothing above helped, try a Safe Mode boot. A Safe Mode boot deletes some system caches thay may help.
Startup your Mac in Safe Mode
A Safe Mode boot takes a longer than a normal boot so be patient.
Once you are in Safe Mode, click Restart from your Apple () menu.
Then try Safari TopSites in full screen mode.
BTW, it's not a bug. No problem using TopSites in full screen mode here.