Scroll changes font weight & text overlaps sidebar

I know this has 2 other mentions, but those solutions seem particular to each issue.
My font jumps from bold to normal on scrolling. Also, with sidebar floated right, text can overlap the sidebar.

If I put a border round the sidebar I can see that the sidebar is scrolling (the border scrolls with the page, leaving the text fixed – I suspect this is the issue?

This is my css:

#sidebar { width: 13%; margin: 0 0 0 12px; padding: 20px 10px 10px 30px; float: right; height: 510px; font-family: 'veteran_typewriterregular', Arial, Verdana, sans-serif; letter-spacing: 1px; clear: both;   }
#sidebar h2 { text-shadow: none; color: #494948; text-align: left; font-weight: normal; }
#sidebar a { text-decoration: none; color:#3b3b3b; font-style: normal; }
#sidebar a:hover { text-decoration: none; color: #558790; font-weight: normal; }
#sidebar ul { list-style-type: none; padding-left: 0; }
#sidebar li { list-style-type: none; border-bottom: solid #fff 1px; }

https://ww.wp.xz.cn/plugins/q2w3-fixed-widget/