By using a custom ViewTabDisplayerUI, I managed to have the HtmlRenderer's _renderPlainString() method produce
NullPointerExceptions when the tabs got too narrow to hold its title. The reason is that
HtmlRenderer#_renderPlainString() uses the result of g.getClip() as an argument to the constructor of Area on line 407:

Shape shape = g.getClip();

if (s != null) {
    Area area = new Area(shape);
    area.intersect(new Area(new Rectangle(x, y, w, h)));
    g.setClip(area);
} else {
    g.setClip(new Rectangle(x, y, w, h));
}

g.drawString("...", x, y);
g.setClip(shape);

In my ViewTabDisplayerUI I paint to the graphics context of a BufferedImage (for transparency effects), and because of
that, no clip is set, i.e. g.getClip() returns null. There's no problem to avoid this issue, but HtmlRenderer should
definitely check if the clip is null.

This might also be an issue in other methods of HtmlRenderer (not tested).