Some function keys switch cases of letters under and after the cursor

Question

Foreword

I use UltiSnips and have the following definitions in my .vimrc file

let g:UltiSnipsExpandTrigger="<F8>"
let g:UltiSnipsJumpForwardTrigger="<F8>"
let g:UltiSnipsJumpBackwardTrigger="<S-F8>"

The problem

The third definition doesn't work; the strange behavior I witness is that ⇧+F8 switches the case of the letter under the cursor and that after it and move to the next char just as I had hit 2~ in normal mode.

Troubleshooting

Mindful of another strange keyboard behavior I experienced in the bash with fzf, I decided to investigate which escape sequence is sent when I press (shifted) function keys, by issuing them in the terminal prepended by ⇧+V. What I "discovered" is that ⇧+F8 sends ^[[32~, which is the same escape sequence sent by Esc[32~, which indeed does the following:

• Esc exits UltiSnips' select mode, thus returning to normal mode
• [3 do nothing since the latter "kills" the former, which doesn't expect a digit after it
• 2~ switch the case of under-cursor and immediately-after-cursor letters and moves to next character

which is what I see happening everytime.

Furthermore, I've checked that function keys from F1 to F4 send ^[[11~ ^[[12~ ^[[13~ ^[[14~ whereas combinations from ⇧+F1 to ⇧+F4 send ^[[23~ ^[[24~ ^[[25~ ^[[26~; based on these escape sequences I'd expect all these keys to misbehave (in the same way too, except for the count), but they don't! Of the aforementioned 8 combinations, only ⇧+F3 switches the case of the following 5 letters (as the ^[[5~ escape sequence implies), and the others do nothing (they don't even exit the select mode!).

Info

I use URxvt, $TERM is rxvt-unicode-256color and Vim version is 8.1.

Related question(s) and pages

The first time I had this problem was long ago and I searched for an answer already; since then I've forgotted I had found a related (actually exactly the same) question but it was and is still unanswered (maybe this is way I've forgotten it).

Here is a related wikipage.

Here is a related issue on GitHub.

Answer 1

The codes which keys produce varies wildly by terminal. vim tries to guess which escape sequences corresponds to which keycodes (<f1> etc) based on the $TERM variable and terminfo. The shifted f-keys are not standardized and they do not have termcap/terminfo entries. In this case, vim falls back to the xterm ones. Note, wherever ^[ is written, this means a literal escape character.

Changing vim's key codes

For many keys, vim has a key code for it of the form <..>. This includes the shifted f-keys, <s-f1>..<s-f37>. The way to change this for your terminal is through set (the following are equivalent):

set <s-f1>=^[[23~
execute "set <s-f1>=\<esc>[23~"

Some keys do not have a named key code, such as <s-enter>. In this case, vim provides many extra function keys, up to <f37> which you can use, if your terminal supports it. As usual, you can grab the key code by typing it after ctrl-v or with cat.

set <f37>=^[OM
execute "set <f37>=\<esc>OM"

Then you can use <f37> in place of <s-enter> whenever you want to use that key.

Explaining the original behavior

Below is a list of the keys in urxvt and xterm, produced by running cat in a shell then typing each key (except for f13/f14 which comes from infocmp).

In urxvt, shift-f1 and shift-f2 are exactly identical to f11 and f12. Therefore, by default, you cannot use both for different things. This also explains why shift-f1 and shift-f2 did not produce the case changing behavior you saw- vim thought they were some other key.

Likewise, shift-f4 in urvt is the same as f14. I'm not sure why but vim thinks this is the <undo> key, which is t_&8=^[[26;*~, close but not quite the same as f14.

Table

              urxvt       xterm

f1           ^[[11~       ^[OP
f2           ^[[12~       ^[OQ
f3           ^[[13~       ^[OR
f4           ^[[14~       ^[OS

f5           ^[[15~       ^[[15~
f6           ^[[17~       ^[[17~
f7           ^[[18~       ^[[18~
f8           ^[[19~       ^[[19~

f9           ^[[20~       ^[[20~
f10          ^[[21~       ^[[21~
f11          ^[[23~       ^[[23~
f12          ^[[24~       ^[[24~
f13          ^[[25~
f14          ^[[26~

shift-f1     ^[[23~       ^[[1;2P 
shift-f2     ^[[24~       ^[[1;2Q 
shift-f3     ^[[25~       ^[[1;2R 
shift-f4     ^[[26~       ^[[1;2S 

shift-f5     ^[[28~       ^[[15;2~
shift-f6     ^[[29~       ^[[17;2~
shift-f7     ^[[31~       ^[[18;2~
shift-f8     ^[[32~       ^[[19;2~

shift-f9     ^[[33~       ^[[20;2~
shift-f10    ^[[34~       ^[[21;2~
shift-f11    ^[[23$       ^[[23;2~
shift-f12    ^[[24$       ^[[24;2~