[jfultz]

A couple of cases I'm aware of...

* Coding ligatures often display as a single glyph (maybe occupying a single-width character space, or maybe spread out over multiple spaces), but are composed of multiple glyphs. The ligature may "look" like a single character for purposes of selection and cursoring, but it can act like multiple characters when subject to backspacing.

* Similarly, I've seen keyboard interfaces for various languages (e.g., Hindi) where standard grapheme cluster rules bind together a group of code points, but the grapheme cluster was composed from multiple key presses (which typically add one code point each to the cluster). And in some such interfaces I've seen, the cluster can be decomposed by an equal number of backspace presses. I don't have a good sense of how much a monospaced Hindi font makes sense, but it's definitely a case where a "character" doesn't always act "character-like".

[dwringer]

I've always felt ligatures that condense two or more glyphs into something that takes up the space of only one in a monospace font are going beyond what a font should handle and into the realm of what an editor should do. I have several such visual substitutions set up in my .emacs but I don't use fonts that do them on their own.

[trealira]

What about ligatures that make ASCII characters display differently when in proximity, but still use the same number of columns?

For example, when == is written, connect them to be a 2 column wide = instead.

Or when === is written, display a three column equals sign, but it's three bars instead of two.