The text in the video seems to bolden when you go to the first character of a line.
(It’s not actually bold but the textbox moves ever so slightly such that it doesn’t line up perfectly with the textlabel)
When clicking off of the textbox the text will go back 1 pixel like nothing changed.
Sadly but there no ways of fixing that.
I had the same exact issue heppening to me…
That just how selected text looks like in roblox.
You may try making some shenanigans with text layers to make it look less weird but other than that no solution.
You may try making richtext fake layer move upward when entering focus but that makes selection with mouse impossible.