nstextview+caret.m

# pragma mark Custom caret

- (void)drawInsertionPointInRect:(NSRect)aRect color:(NSColor *)aColor turnedOn:(BOOL)flag {
    aRect.size.width = self.caretWidth;
    [super drawInsertionPointInRect:aRect color:aColor turnedOn:flag];
}

// This is a hack to get the caret drawing to work. I know, I know.
- (void)setNeedsDisplayInRect:(NSRect)invalidRect {
    invalidRect.size.width += self.caretWidth - 1;
    [super setNeedsDisplayInRect:invalidRect];
}

Can confirm this works. I know, I know.

Swift version:

override func drawInsertionPoint(in rect: NSRect, color: NSColor, turnedOn flag: Bool) {
    var rect = rect
    rect.size.width = customCaretWidth

    color.set()
    let path = NSBezierPath(roundedRect: rect, xRadius: customCaretWidth / 2, yRadius: customCaretWidth / 2)
    path.fill()
}

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