LESS Builder: neverending Working

see

also

in short: was (almost) fixed in 5.1.1 (although if you rely a lot on Tinymce you may want to use workarounds to 5.1.0 outlined in the Github issue and stick to 5.1.0 since 5.1.1/5.1.2 suffer of a Tinymce update with several side effects)