In summary: Ideally, combining marks should be non-spacing, that is, have a width of 0. This is, however, not an ideal world, and some apps require all glyphs – including combining marks – to have the same width for the font to be considered fixed-pitched/monospaced/non-proportional. Using feature code to reset the width of those glyphs at the layout stage back to 0 is the common trick that works with most apps.