Earlier quoted context omitted.
None of the 3 things you said are true. I recommend you get some experience in rendering before you mislead people too much with these kinds of comments. In reality the problem is trivial, you set up a scissor rect (or explicitly mask the pixels in your shader) and then render only stuff overlapping that square. You don't need to invert the pixels for it to be fast; you can render an arbitrarily nice cursor effect.
I am not sure I understand why do you need clipping at all to render rectangular caret bar (note: cursor is a different entity in UI professional jargon). What exactly you want to be clipped out?
I am assuming that your caret bar may be overlapping text in some way, or that there is a background bitmap that you might be alpha-blending against, etc. Basically I don't want to make an assumption that might break if the UI gets nicer. The case of a strictly opaque strictly rectangular non-antialiased non-smoothly-moving bar does not seem very interesting or nice-looking.