Issue when adding text to existing SRTF copy

Hello,

I’ve found that if I’ve marked some text as screen reader text and then try to add more copy within that text, each new letter entered is surrounded by a new span tag within the original text-format-sr-only span.

A quick test shows that most of the other formatting options from the block pull-down work fine (i.e. text added within the created markup does not add additional markkup) while a few others have the same unwanted behavior, e.g. highlight.