I’m still using LyX 2.3.1 on one PC. There appears to be a bug on Win 10 64bit, latest version. If I insert \oiint in a math expression, nothing is displayed.
[cid:image001.png@01D4ACF6.B3925130] However, the preview works: If I click outside of the math expression, I get the expected result. [cid:image002.png@01D4ACF6.B3925130] -B
