Viewport sliders don't properly update size when dragged manually

[KoBeWi]

Godot version:
3.2 beta4

Issue description:
When you pan viewport outside view, the slider will shrink, but it won't expand again when dragged manually, which later results in the grabber "teleporting" on zoom.

Here's how it works in GIMP:

[KoBeWi]

Still valid in 4206dc6