strange scrolling behavior when toggling on-screen-keyboard visibility

[helguson]

• Device: FP2, bluetooth keyboard
• UT channel: 16.04 rc
• UT build: 2018-W29
• terminal-app version: 0.9.0

Steps to reproduce

1. start terminal-app and enter passcode
2. execute some commands that create a lot of output e.g., nano -h to fill the screen
3. make on-screen-keyboard disappear by swiping it down or switching to another app
4. make on-screen-keyboard appear again by taping the keyboard button
5. start writing a new command
6. press backspace

Expected behavior

• on 3. command line sticks to lower border of available screen
• on 4. command line sticks to lower border of available screen
• on 5. the written symbols appear in the visible command line
• on 6. the last symbol disappears in the visible command line

Actual behavior

• the beginning is normal
  
  
• on 3. command line stays in its position creating empty screen space underneath
  
• on 4. command line jumps a few lines down ending up invisible behind the on-screen-keyboard
  
• on 5. the screen stays in this position; the command is written without being visible
• on 6. the command line jumps back over the lower border of the screen
  

Problem

• scrolling without the user's intention and command line hiding is unpredictable behavior that hinders working with the terminal-app

Additional observations

• taking a screenshot after 4. makes the command line appear already on 5.
• after 3. the current position marker is empty