Alacritty has supported mode 2026 for synchornized updates for a few
months, but the terminfo entry still used the old DCS sequence
originally supported by iTerm2. Since many other terminal emulators and
applications seem to be standardizing around 2026, change the terminfo
entry to use SM instead of DCS.
This adds documentation stating that Alacritty supports the synchronized
update escape, which was implemented in 9575aed.
Since tmux does check terminfo for this feature, the `Sync` capability
has also been added. Tmux's implementation can be found here:
f5b7ebc540/tty-features.c (L185)
This adds support for double underlines using the colon separated escape
sequence `CSI 4 : 2 m`.
Alacritty will now also always fallback to the normal underline in case
any of the other underlines like the undercurl are specified. The escape
sequence `CSI 4 : 0 m` can now be used to clear all underlines.
Some terminals support `CSI 21 m` for double underline, but since
Alacritty already uses that as cancel bold which is a little more
consistent, that behavior has not changed. So the colon separated
variant must be used.
Ncurses does some special handling for backspace, normally comparing it
to the KEY_BACKSPACE constant. However to make this work, the right
backspace escape sequence needs to be set in the terminfo.
XTerm has this set to ^H, but it always sends ^H. Since Alacritty
usually sends ^? instead, this needs to be specified to allow correct
interpretation for ncurses.
