Extender part `uni0305.s00` has endConnectorLength 0

[mkorje]

The glyph for U+0305 has glyph parts in the GlyphAssembly table in MATH to allow for constructing an overline accent of any size.

Currently, Noto Sans Math has two glyph parts with the glyph uni0305.s00. The first is not an extender, and has startConnectorLength 0 and endConnectorLength 417. The second is an extender, and has startConnectorLength 417 and endConnectorLength 0. The second, as it is an extender, is what gets repeated when constructing a larger sized accent. But because the endConnectorLength is 0, the part is then taken to not have a connector on that end. The spec mentions the below:

If the part does not have a connector on one of its ends, the corresponding connector length should be set to zero.

[simoncozens]

To be honest I think the reason SILE, Firefox and LuaTeX get this right, and Typst does not, is because Typst is not correctly interpreting the spec.

But because the endConnectorLength is 0, the part is then taken to not have a connector on that end.

Right, but we've also said that the whole glyph is a connector on its left side. "Connector" here just means "a straight bit you can overstrike".

What the values say is: You set uni0305 first. uni0305 has 417 units of straight line (the whole thing) on its right side, so you will be fine if you overstrike the next glyph by any amount between 0 and 417 units on the right hand side; then you set uni0305.s00 next to it as many times as you like, and to make it fit the width, you can eat up any amount of space from the previous glyph between 0 and 417 units. In other words, the whole thing is a straight line and you can overstrike it wherever you like.

I see in typst/typst#5283 that there are gaps between the overlines. But step one of assembling glyph assemblies is to set the glyphs with maximum overlaps and then extend the connectors until you reach the desired width. There's no way that glyphs with zero sidebearings should ever produce gaps.

[khaledhosny]

I agree this is a bug in Noto Sans Math (the end connector should not be zero), but I also think Typst is doing something wrong here. I’m going to fix the end connector either way.