This issue affects Windows builds and has been reported on GitHub as Issue #523
For now it can be solved by editing synfigstudio.bat (in Synfig’s folder)
add
set GTK_CSD=0
before
cd "%~dp0\"
start "" "%~dp0\bin\synfigstudio.exe"
then start SynfigStudio from synfigstudio.bat
You can modify your desktop shortcut