Issue with visible tabs
When asked to render spaces visibly with the show attribute, pango produces glyphs with PANGO_UNKNOWN_GLYPH('\t'), with a width that varies depending on the placement. pango's unknown glyph drawing code knows how to render these properly. But the GTK glyph cache will only cache a single rendering (well, one per subpixel position) for this glyph, the first time it sees it.