Go to Edit > Preferences... Go to Appearance > Fonts. Click and hold the mouse button on "Always use my font settings..." check box, move the cursor to an unchecked radio button above it (in my case Serif was checked, Sans Serif was unchecked, I released the mouse cursor on Sans Serif). Two radio buttons will now be checked. Clicking and dragging from almost anything and releasing it on a radio button or checkbox will alternate its checked state.
dup of 35606...this should be fixed now, try it on a new nightly and reopen that if it's not *** This bug has been marked as a duplicate of 35606 ***
verified duplicate. In a current build, this is fixed (including both the onmouseup state change, and the opportunity to get two radio concurrently selected.) [Although it looks this was fixed the day after the m16 branch was cut, so it may still work this way in M16).