This is likely caused by a regression in TinyMCE (which was updated in 4.9.6).
See https://core.trac.wordpress.org/ticket/44134 for the fixes to go into 4.9.7.
This is likely caused by a regression in TinyMCE (which was updated in 4.9.6).
See https://core.trac.wordpress.org/ticket/44134 for the fixes to go into 4.9.7.