Double clicking on a screenshot taken doesn't show it's full size image in a new window

**Game information**
This is about the steam screenshot tool that appears after a user has taken an ingame screenshot.

**Distribution name and version where applicable**
Fedora 30 Workstation

**Flatpak info**
Flatpak 1.4.3
nvidia-430-40
default

**Problem description**
The flatpak version of steam is unable to show the screenshots taken in their full size in a new window likely due to sandbox.

**Does this issue reproduce with native Steam**
No.