I have no clue why my previous attempts at customizing drawInsertionPoint(in:color:turnedOn:) always produced visual glitches. I really tried a lot of different ways. But it turns out you don’t have to do that much, really:

class MyTextView: NSTextView {
    var caretSize: CGFloat = 4
    open override func drawInsertionPoint(in rect: NSRect, color: NSColor, turnedOn flag: Bool) {
        var rect = rect
        rect.size.width = caretSize
        super.drawInsertionPoint(in: rect, color: color, turnedOn: flag)

    open override func setNeedsDisplay(_ rect: NSRect, avoidAdditionalLayout flag: Bool) {
        var rect = rect
        rect.size.width += caretSize - 1
        super.setNeedsDisplay(rect, avoidAdditionalLayout: flag)

Adapted to Swift from a Gist by koenbok.

That’s all it takes. Marvelous!

Update: So it turns out that the drawing works just fine in a test project, but I end up with 1px-wide insertion points/carets/I-beams when I click with the mouse or press up/down arrow keys. Left and right work fine. I wonder what’s wrong with this. Isn’t it supposed to be a rather simple customization?