Not sure how much work would be needed to fix this, or if it is at all something you would want to do. But I do get a little frustrated at times, when words or sentences I’ve decided to underline are also underline because of spelling issues (like the word simply not being recognized). Now, I usually write either fantasy and such, or stuff related to sexology, which means I often use words and names that are not recognized by the app, regardless of what dictionary I use. Many strange words… So I guess I see this more often than some other users (assumption on my part).
What I would like to see is basically that the underline set by the user will not be on the exact same pixel-row as the “misspelling line” (a.k.a. “the red line”) as the user set underline will always end up disappearing. Besides it looking horrid (I know pretty much every other editor will do the same thing, though), it makes it difficult to find these parts of the text when looking for them (which is, when writing notes and research and such, one of my more common practices - sadly).
But I mean, just having them separated by one or two rows of pixels, so that the user set underline is also visible would be a truly great thing. If at all possible.
I recognize that this would be a problem trying to solve in relation to format settings and so on, and I have no idea how much programming would be needed. But it is something I certainly wish for. Quite often, I must confess.