The button fonts with Windows L&F looks strange (e.g. handles on dockables in Isabelle/jEdit), see Isabelle/99da14fa28b8.
This problem showed up first in jdk-11.0.6, and was the reason to ship Isabelle2020 with jdk-11.0.5, see Isabelle/73d1dc57215f.
Isabelle2021 will use jdk-11.0.9 (or later), so this needs to be addressed somehow.