Including bell character in fish_prompt messes up command line rendering

[ftilde]

Minimal config.fish to reproduce:

function fish_prompt
  printf ">\a"
end
function fish_user_key_bindings
  fish_vi_key_bindings
end

First notice that the prompt contains a space after the >.

Now enter some text, press esc to enter normal mode. Notice how the entered text appears to jump to the left and is now directly adjacent to the >. The cursor position is still related to the text (shifted to the right, i.e. including space after >) and any edits seem also to correspond to the (shifted to the right) text.

The issue is present on current master (4227f53).