Just installed the RC of LibreOffice 220.127.116.11 and tried importing a simple MathML formula. Then tried editing/inserting an element but on the Elements sidebar the Binary operators were selected so I wanted to change it to e.g. Attributes. Normal behaviour would be: click on dropdown, the elements refresh and show the selected group of elements. But nothing happens! However if I use cursor keys while the dropdown is selected - the selected elements group DO show. So it must be a bug with non registering the mouse click while selecting the Elements group... The context menu in the formula editor works just fine.
Thanks for reporting. Known bug, already fixed in RC2.
Best regards. JBF
*** This bug has been marked as a duplicate of bug 96119 ***