Chat input field height is 1 line behind actual line count

Versions

• OS: Windows 10 (x64)
• Gajim version: 1.12
• GTK version: >= 3.22.30
• Python-nbxmpp version: >= 0.6.6

Steps to reproduce the problem

1. Open chat
2. type regular long message (with spaces and no line breaks) that exceeds the width of the input field

Expected behavior

• Input field expands in height to fit the lines of the entered message

Actual behavior

• Input field expands BUT is always a line behind. Means:
  • 1 line message = 1 "height" of input field (obviously okay)
  • 2 line message = 1 "height" [sig] of input field (here it start to fall apart)
  • 3 line message = 2 "height" of input field
  • ... so on, till the input field reaches its maximum it will expand to.
• The problem gets solved very temporarily (per typed message) when you go up with the cursor until a line that was out of view so far (and down again). It also fixes the upcoming lines. Let's say your message got split into 3 lines (of which you see by now 2 lines).
  1. go up one line: still in view, nothing happens.
  2. go up another line: the input field expands "because" the line was out of view.
  3. now the whole message is visible and stays visible even you go down with the cursor again to the last line
  4. go on typing till message gets split up into a fourth line
  5. input field expands as expected till the message gets send or reaches the maximum of 5 lines, then a scroll bar appears. 5 with the next message the game starts over

Note

This is pretty similar to #8980 (closed) but actually it is not.