background-image of transparent image previews doesn't repeat and gets stretched

[EtzBetz]

Steps to reproduce

1. Upload transparent image to nextcloud
2. Open the uploaded image in the image preview
3. Hover over the image to enable the background-image

Expected behaviour

The transparent part of the image should be covered by those squares that indicate transparency (I did view this by disabling background-size in the CSS inspector, but that only fixes it until you hover again):

Actual behaviour

The resource-image is stretched as big as the image is:

Server configuration

Operating system: Ubuntu Server 16.04 LTS

Web server: Apache 2

Database: MySQL 5.7.20

PHP version: 7.0.22

Nextcloud version: 12.0.4

Updated from an older Nextcloud/ownCloud or fresh install: Yes, updated from some old ownCloud to nextCloud 9/10, then 11, 12 and now 12 > 12.0.4.

Where did you install Nextcloud from:
nextcloud.com

Client configuration

Browser: Chrome 62.0.3202.94 and Firefox 57.0.2

Operating system: macOS 10.13.1 (17B1003), Windows 10 1709

[MorrisJobke]

looks like a gallery thing.

cc @pixelipo @skjnldsv @oparoz

[oparoz]

Would probably be best to open in nextcloud/gallery

[MorrisJobke]

Fix is in nextcloud/gallery#341

[MorrisJobke]

Will be fixed in 13.