Mysterious blank page with uBlock extension

We got report that some people got a complete blank page when visiting our site using uBlock extension.

I started to investigate this report. First thing I wanted to do is to reproduce the issue. I started by installing uBlock extension and then visit our site, but it worked perfectly. I couldn’t reproduce the issue. How strange! But it was also a good sign that not everyone were affected by this issue.

Why did only some people get a blank page?
Some of my colleagues did get this issue and some did not. So why? We started to debug on my colleague’s computer, which experienced this issue.

We opened uBlock logger tool and looked for what filter rules that caused blank page.

“#root > div[class]:not([id]):first-of-type” and indeed removing that element will cause blank page, but still I couldn’t see that filter rule was being applied on my computer. So Why?

It turned out that this filter rule was only on a specific filter list, “SWE: Frellwit’s Swedish Filter”. For some reason this filter list was activated by default for only some users.

To make a change to “SWE: Frellwit’s Swedish Filter” was quite easy. This filter list was on a Github repository (https://github.com/lassekongo83/Frellwits-filter-lists) and I filed an issue (https://github.com/lassekongo83/Frellwits-filter-lists/issues/97) on their repository describing the affected filter rule with a proposal fix. The owner accepted the proposed fix quite fast and the blank page issue was resolved!