The background color is not painted properly in a TextPane with multiple spans. Here's a simple example:
<TextNode text=" some random text "/>
"><TextNode text="(some remark)"/></Span>
If the window is wide enough, you'll see an extra splotch of yellow to the right of the text, and part of the text will not be yellow. If you reshape the window to be narrower than the text, you'll see many variations of blocks of yellow showing outside the text, and parts of the next not having a yellow background.
(The bold and italic styles make it interesting, but you can take them away and still see the bug, just as long as the paragraph has multiple spans. And the reshaping bug doesn't even require multiple spans.)