I know this thread is old (over 1 year now), but I have finnaly found solution to this problem. There are several limitations (I explain later).
So the actual behavior as of 17. November 2018 is as follows: Raspbian Lite disconnected from network works as expected. User connects to Internet and about 15 seconds after systemd completes bootup and asks for login, cursor jumps at the beggining of the same line. This results in broken input text and if user is fast enough it intercepts password input. No matter what connection interface does user choose as it is proved it affects both onboard WLAN and/or Ethernet. *Tested on RPi Zero W.
The explanation (I am no linux developer or programmer, so I try to explain as good as I can.) : Very similiar behavior was observed on Ubuntu. As stated here: https://unix.stackexchange.com/question ... ring-login
the problem comes from systemd module named systemd.fsckd. This module is responsible for launching fsck by statements made in /etc/fstab. As stated by ShiftPlusOne in mailing lists message - this module is actually a piece of trash and should not be in systemd as it not only causes erratic behavior, but is also a code bloat. As stated by ShiftPlusOne at https://github.com/systemd/systemd/comm ... 76c4c253ec
this module was eventually removed from systemd, because reasons above. This would have fixed out problem, however Debian decided on keeping it here. The updated version sadly did not reach us and probably never will. In desperate search of fix, I have came around this https://unix.stackexchange.com/question ... ring-login
thread and as stated by Jeff Schaller: "so I could see that the systemd-fsckd was somehow responsible. To prove it, I turned off file system checking in my fstab, and the problem went away." First I had to open /boot/cmdline.txt, removed parameter "fsck.repair=yes". Second I opened my /etc/fstab file and looked for sixth column (this one is used to specify fsck at boot time). I could see it contained "2" for boot mountpoint and "1" for root filesystem. So I changed these two to "0" did a reboot and voila! The problem went away.
I you apply my fix, you will lose ability to recover from sudden power loss. This is because this fix only disables daemon for fsck repair upon power up. By sudden power loss your RPi SD Card may become unbootable and in worst scerario the filesystem get corrupted. I AM NOT RESPONSIBLE FOR DATA LOSS.
If you experience power loss and able to successfully boot up, do not forget to run "fsck.ext4 -f /dev/mmcblk0p2" as superuser to fix errors on root partition.
Issue commands as superuser.
1. "nano /boot/cmdline.txt"
2. Search for "fsck.repair=yes" parameter and remove it. Exit and save file.
3. "nano /etc/fstab"
4. Search for expression "0 2" and "0 1". Change both of them to "0 0". Exit and save file.
5. Reboot and done.
6. The cursor should no longer jump on beggining of login line.
The reason Raspbian Lite Jessie was not affected were most likely due to less aggressive enforcement of fsck at boot time.
Feel free to correct or enhance my answer.
I even think of writing simple script to make it little more automated. I will reply back if I succeed.