Steps to reproduce :

1) Download attached project "Test Project 3.zip" and open in Unity.
2) Open scene "scene".
3) Check Game tab. Notice there is no text inside Scrollbar(Its above of it) see attached image "Above.png". Press Play mode and you will see the text appeared in Scrollbar.
4) Stop play mode and text will slide above of Scrollbar again.
5) To make text appear inside Scrollbar while not in Play mode, in Hierarchy tab click on Canvas >> Scroll View >> Viewport >> Content >> EDIT ME.
6) Edit text in Inspector tab and press Apply. Click Ctr+s and text will appear in Scrollbar. (see attached image " inside.png"

Note: As soon as you will edit this text next time and press Apply, text will slide above again and you have to press Ctrl+s to make it visible in Scrollbar.

Reproduced with : 5.3.5p1, 5.3.5p4, 5.4.0b22.

Not reproduced with : 5.2.4f1.