Forward delete not working with st terminal

With st terminal (using bash or zsh), forward delete does not work, but prints [P instead.

The st FAQ recommends adding set enable-keypad on to ~/.inputrc. If this is added, st sends ^[[3~ (and otherwise ^[[P) which makes delete work in normal use of the terminal. Prompt-toolkit seems to always receive the ^[[P sequence, however.

One workaround is to add a binding for this key sequence to prompt-toolkit, see for example ipython/ipython/issues/12567 where this problem was reported before.

Another workaround, also mentioned in the st FAQ, is to use tput smkx to enable keypad mode. This also seems to work with prompt-toolkit, but only until a pager like less is used.

Although st behaves differently than all other terminals, this behavior seems to be deliberate. For much more context related to this problem, see for example fish-shell/fish-shell/issues/2139.

Start by tracing prompt-toolkit's handling of the ^[[P sequence and compare it with the key-sequence binding workaround described in the issue. Reproduce the forward-delete behavior in st using bash or zsh, then verify that forward delete works through pager use without requiring a user-configured workaround.