ucss breaks sticky header

I am not 100% percent sure, but the reason why your sticky header doesn’t work anymore, is that this sticky function is Javascript driven and not by CSS. Cache plugin for WP can only strip CSS vom CSS files and not if CSS is generated by Javascript.

the sticky header stops working when i use ucss, so there must be some css being removed.

No, CSS is not removed. CSS class has to be added on the fly while user action and to get this work Javascript is needed. UCSS function neither simulates user actions like scrolling nor can parse Javascript. This is not a bug or malfunction. It is typical for each other or similar functions that can make UCSS available. You must find one who simplifies this faked sticky function with pure CSS.

So I tried purifycss.online, and when I use the generated css file the sticky header is working fine. So there really must be some selecter to whitelist.

I’m to stupid to find out ??

Again, find one who simplifies this sticky header without Javascript, what the better way for sticky position is than Javascript or disable UCSS function. If cache plugin for WP offers a high number of optimizations it doesn’t mean, that each optimization works in all combinations with all plugins or themes.

I found what to whitelist to make the sticky header work on blocksy theme.

Whitelist : .ct* end *data*

All working great and saved another 30kb.