rendering artifacts (black triangle) in some long articles

# Issue details

### Duplicate?
Have you searched the issues of this repository if your issue is already known? yes

### Actual behaviour
Some long articles (example: ) have pieces rendered incorrectly in the app, see the screenshot below: Somewhere far down, a black triangle (always that shape) is overlayed over the text, and text from somewhere else in the article too, which makes reading impossible. Marking the text makes it barely usable.


# Environment details
- **wallabag app version**: 1.0.2 (106)
- **wallabag app installation source**: f-droid
- **Android OS version**: 7.1
- **Android ROM** (e.g. stock, LineageOS, SlimRom,…): custom, LineageOS
- **Android hardware**: golden / Galaxy S3mini
- **wallabag server version**: 2.3.1

# Your experience with wallabag Android app
Have you had any luck using wallabag Android app before?

Yes, works great, thank you!

Start by reproducing the reported rendering artifact in the wallabag Android app using the linked long article on Android 7.1 with app version 1.0.2. Compare the affected article with shorter articles and use the screenshot as the expected symptom. Done means the article no longer shows the black triangle or unrelated overlaid text and remains usable when selecting text.

Written by the indexing model from the issue text.