Easymotion breaks my undo history

[eliasnystrom]

For some time I've had the problem that my undohistory breaks spontaneously, seemingly at random. Just now I found that the reason for this is that the undolevel is set to -1. EasyMotion is the only one of my currently installed plugins that touches this setting (and it sets it to -1).

Why does EasyMotion do this? So far I haven't been able to detect any particular pattern behind the occurrences. From the code it seems to be part of some fail handling.

[haya14busa]

Hmm.... today, I also had the problem, I cannot reproduce it though. I'll look into it.

[damage220]

I have the same issue:

1. Open a file.
2. Search through easymotion.
3. Type "u" for a few times.
   Tab has changed its color and I can't leave the editor - "No write since last change for buffer".

[haya14busa]

I didn't experience this problem thesedays, and I cannot reproduce this.
Please reopen if you find reproducible way.

[dimhatz]

I also experience this issue. GVim on Windows 10, both 64bit. Precompiled version from https://tuxproject.de/projects/vim/

VIM - Vi IMproved 8.0 (2016 Sep 12, compiled Jun 22 2017 20:05:01)
MS-Windows 64-bit GUI version with OLE support
Included patches: 1-655
Compiled by hp@HP-PC
Huge version with GUI. 

Also tested 64-bit version v8.0.0698 from https://github.com/vim/vim-win32-installer/releases.
Bug still occurs. Haven't tested other platforms.

Here is how to reproduce.
Install the plugin and put this in _vimrc:

set nocompatible               " Be iMproved
set laststatus=2    " Always show status bar
map <Leader> <Plug>(easymotion-prefix)

Now open any file that has some text in it
:e ~/_vimrc
Press <Leader>j to highlight the beginning of each line.
Press <Esc> to cancel the motion (or any key to perform motion - does not matter)
Press u to undo.

Expected behavior: nothing happens to the buffer, since we did not change any text.

Actual behavior: u shows the buffer in a changed state (as if some change was performed). The statusline on the bottom shows [+] as if there was a modification to the buffer. This causes confusion when using undo, I'm not sure if it also corrupts the undo history.

P.S. using the original prefix mapping <Leader><Leader> still causes the bug to appear.
P.P.S. opened as new issue here: #339