Nemo: Blank squares on side bar

[fcastilloec]

```

• Nemo version: 3.4.7
• Is issue with desktop or windowed nemo? windowed nemo
• Distribution - Ubuntu 16.04.3
• nVidia GTX-1070 using nvidia drivers v384.59 from ppa:graphics-drivers/ppa
• 64 bit
  ```

Issue
The sidebar is displaying blank sections on the top or bottom. Depending on where the scroll bar is, this can be made more obvious when scrolling.
This GIF shows the blank section, both at the top and/or bottom, depending on the scrolling. I've also added a GIF that shows when you try to keep scrolling past the end.


On previous versions, there was no blank square.

Steps to reproduce
It should be visible immediately, but scrolling makes it more obvious
Here's a GIF of how it looks when scrolling.

Expected behaviour
The blank areas shouldn't be there

Other information
I think something similar is happening with other themes, as reported in issue #1582 but that issue was closed since the blame was passed to the arc theme instead of nemo itself. The default Ambiance theme hasn't been updated, only nemo, so nemo is the one that broke it.

[fcastilloec]

Closing the issue without any explanation, or a comment or anything???????
Can somebody care to explain what's going on?

[leigh123linux]

We don't care about issues caused by use outside of cinnamon!

[fcastilloec]

Oh! I see... I guess you have a point.... Sorry about that. I should have known before posting it

[leigh123linux]

It actually looks like the theme has broken overshoot scroll

https://zoltanp.github.io/2017/06/gtk3-scroll-fix.html

[dominichayesferen]

Yeah, I was going to mention that that usually means the GTK has a missing overshoot image...

[brownsr]

If this is using one of our themes then please say which one.

[dominichayesferen]

@brownsr That looks like stock Ubuntu Ambiance...

[leigh123linux]

This is what the overshoot scroll is meant to look (arc-theme dark).