Auto lock lines/highlighter etc. to horizontal or vertical
Add a tool/option to force the highlighter to adhere to text.
I agree with this opinion. It is very beneficial.
highlight only in text, no free hand !
Add a feature for the highlighter where you can highlight text with a horizontal line instead of having to do it free hand.
In pdfs, mark the text highlighting in straight lines, as on ezpdf. Now I draw with the pen over the text, but I usually go up and down, not straight.