control character display.

[adamyg]

Alt-Q and other functions which may display control-characters msibehave at time, for example:

Alt-Q should insert a literal horizontal tab in the buffer. Instead it inserts "\x09" (including the double quotes).
It probably does the same for other characters, e.g. and so on.

Presentation of control-characters is dependent on whether or not a the display is Unicode font.

Shall test non-printable logic; tabs for example should be either Unicode-Tab or hex
Current cmap display control-characters as Unicode: 2400-2401F, yet these are not always available.

[adamyg]

linux/bsd/solaris: fc-list and fc-query, examples:

 fc-list :charset=1f63f
 fc-query /usr/share/fonts/truetype/dejavu/DejaVuSansMono.ttf

 General ability to set, yet no standard means to determine active font. cmdline options cleanest. 
 https://github.com/wez/wezterm/issues/4181
 vttests/query-fonts.pl, uses extend "CSI > Ps; Ps T" to query fontname in hex

win32: GetFontUnicodeRanges

minitty and others? https://github.com/mintty/mintty/wiki/CtrlSeqs

 Font glyph coverage enquiry
 Fonts vary widely in their Unicode coverage, i.e. they usually miss glyphs for many characters. 
 The following sequence can be    used to enquire about support for a specified list of characters.

    ^[]7771;?;char0;char1...^G

 Characters shall be specified with their decimal Unicode codepoint. Any number of characters can be given.
 Mintty replies with the same sequence, except that the question mark is replaced with an exclamation mark 
 and that codes for characters that the current font does not have a glyph for are omitted.