trouble with mappings that use Alt/Meta

[shticktical]

vim

In vim(8.2) insert mode 'Alt+b' goes backwards but 'Alt+f' does nothing. More annoyingly 'Alt+b' 'Alt+U', undoes all the line changes instead of expected behavior to capitalize the last word.
How can I fix this?

Neovim

In neovim 0.5.0 everything works as expected with a very annoying side effect: quickly exiting insert mode and undoing the change (<C-[> u) will not exit insert mode but instead will capitalize word under cursor. I suspect this is because nvim doesn't distinguish between + and Alt+something.
What setting/mapping I could change to have ESC+something stop being interpreted as Meta+something in neovim?

[jonhiggs]

How can I fix this?

The code is fairly simple. Throughout the plugin/*.vim files you'll find calls to the feedkeys function. It basically replays a set of keystrokes verbatim to make the desired transformation. To fix it, you would find a better set of keystrokes to feed in.

You could also try calling the function directly with :ReadlineForwardWord to see whether the problem is with the binding or the function.

a very annoying side effect: quickly exiting insert mode and undoing the change (<C-[> u) will not exit insert mode but instead will capitalize word under cursor.

I'm running Neovim 0.5.0 too, and I'm not able to reproduce this problem but perhaps I'm not following your instructions correctly.

Can you please describe again your method to reproduce the second problem?