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; Update TODO item about ligature support

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* etc/TODO: Add the todo item for moving cursor "inside" a
ligature.
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Eli Zaretskii 2020-05-19 18:46:27 +03:00
parent 03d44acfdd
commit 4b9fbdb5a7
1 changed files with 31 additions and 0 deletions
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@ -264,6 +264,37 @@ of the mode line.
The prettify-symbols-mode should be deprecated once ligature support
is in place.
A related, but somewhat independent, feature is being able to move the
cursor "into a ligature", whereby cursor motion commands shows some
pseudo-cursor on some part of a ligature. For example, if "ffi" is
displayed as a ligature, then moving by one buffer position should
show the middle part of the ligature's glyph similar to the cursor
display: some special background and perhaps also a special
foreground. There are two possible ways of figuring out the offset at
which to display the pseudo-cursor:
. Arbitrarily divide the ligature's glyph width W into N parts,
where N is the number of codepoints composed into the ligature, then
move that pseudo-cursor by W/N pixels each time a cursor-motion
command is invoked;
. Use the font information. For example, HarfBuzz has the
hb_ot_layout_get_ligature_carets API for that purpose. However,
it could be that few fonts actually have that information recorded
in them, in which case the previous heuristics will be needed as
fallback.
One subtle issue needs to be resolved to have this feature of
"sub-glyph" cursor movement inside composed characters. The way Emacs
currently displays the default block cursor is by simply redrawing the
glyph at point in reverse video. So Emacs currently doesn't have a
way of displaying a cursor that "covers" only part of a glyph. To
make this happen, the display code will probably need to be changed to
draw the cursor as part of drawing the foreground and/or background of
the corresponding glyph, which is against the current flow of the
display code: it generally first completely draws the background and
foreground of the entire text that needs to be redrawn, and only then
draws the cursor where it should be placed.
** Support for Stylistic Sets
This will allow using "alternate glyphs" supported by modern fonts.
For an overview of this feature, see