fci-mode and line-spacing

[cpence]

This may be related to #15, but if line-spacing is non-zero, gaps appear in the FCI between each line of text. (I really like (setq-default line-spacing 2), as it opens up the vertical display of text on the screen.)

Is there either a fix, or another way to specify looser leading that wouldn't clash with FCI?

[alpaker]

This is unrelated to #15, and unfixable given the current capabilities of Emacs's display engine. As currently implemented fci-mode doesn't draw a single line; the fill-column rule is actually a series of independently-drawn line segments, one per line. This interacts in an unfortunate way with the display engine's line-spacing feature, which has a very straight-forward implementation and, essentially, works like this: Each line in a window is rendered in a fashion that's agnostic wrt to the value of line-spacing. Then, if line-spacing is non-zero, that many pixels are inserted between lines and filled with the appropriate background color for that region. That's why, with non-zero line-spacing, the fill-column rule appears fragmented: a given rule segment is part of the content of a window line, and the leading introduced by non-zero line-spacing is, by definition, inserted between lines.

AFAICT there's no way to insert anything in the gap between lines that's opened up by a non-zero value of line-spacing. The display engine is organized at a deep level around line-based display (because Emacs is supposed to be fundamentally agnostic between character and graphical terminals, and the former are necessarily structured in terms of text lines); anything you ask it to render will be rendered as part of a line, and so can't encroach into the space opened up by leading.

The natural step at this point is to consider an implementation that doesn't participate in line-based rendering, such as laying a single background image over the entire window. Unfortunately, the only way to do at the moment is to specify a stipple image for the default face, which only works on the X (theoretically it's also supported on OS X, but I've never been able to get it to work on that platform).

Sorry I can't be more helpful. I've been aware of this problem for a while (I should add it the Known Issues section) and have looked into possible fixes, but to no avail. I'd love a solution (I use a line-spacing of 1 in my normal setup) and if anyone has suggestions I'd welcome them.

[cpence]

Awesome and detailed reply, thanks! This is why I've never had the guts to jump into Emacs development. =)

[alpaker]

Just FYI, I'm abandoning this package and welcoming requests to be take over as maintainer.

[alpaker]

The functionality provided by this package has now been implemented natively in Emacs as display-fill-column-indicator-mode, available as of version 27.0.90. Unless you are forced to use an older Emacs, you should use the native implementation instead of fci-mode. The former is unaffected by most of the compatibility issues that this add-on has.