I was able to re-create the functionality on the Demo page: https://www.tiny.cloud/docs/tinymce/6/
Start selecting the text below the editor Selecting Text
make sure your mouse is still down while hovering over the tinymce editor Still holding cursor down
and then release - and you cursor is now in the tinymce editor while the text is still selected. Released cursor over TinyMCE, arrow to cursor
This has caused issues where the users have gone back to re-select the content in another input element, but that text is already selected - so they drag/drop the text in tinymce (which they don't want to do).
Select of the other element stays, cursor does not enter TinyMCE.