Using commandline when trapping a signal in background

[iGr33k]

~> fish --version
fish, version 3.6.1
~> echo $version
3.6.1
~> echo $TERM
alacritty
# OS is Manjaro with with i3 window manager.

Tried running with sh -c 'env HOME=$(mktemp -d) XDG_CONFIG_HOME= fish' - produces the same behavior.

When in background, commandline "<cmd>" inside a function that handles signals doesn't actually insert text into the prompt.

Let's open two terminals:

# in the first one
function testfunc --on-signal SIGUSR1
   commandline -r "ls -la"
end
echo $fish_pid # xxxx

# in the second one
kill -SIGUSR1 xxxx

Produces no commands inserted into the first terminal. However, when sending the signal from the first terminal to itself, everything works as expected.
Furthermore, commandline -f execute would insert a newline as expected, and eval "ls -la" works as well.

[zanchey]

The only other version I have to hand is 3.0.2, where this works as expected. It's broken in 3.6.1 and in HEAD.