Create a new filter class for element hiding emulation filters

[fhd]

Background

Element hiding filters hide elements specified via standard CSS selectors. Standard CSS selectors are however not always powerful enough, hence we extended the syntax with CSS property filters, which are element hiding filters that can also match elements based on the CSS properties they have. #3143 aims to further extend the syntax by supporting the new CSS4 :has() pseudo class.

We need a single filter class for all such non-standard extensions to the element hiding syntax because those extensions need to work in conjunction, they cannot be mutually exclusive.

They are quite similar anyway: They do not get treated like other element hiding rules, they need a content script to actually apply them. Because of this, they add runtime overhead and should never be generic (i.e. not domain-specific).

The new filter class should have flags to mark which extensions are actually used in which filter, to make it easier for the content scripts to apply them.

The best name we could come up with for this type of filter is Element Hiding Emulation Filter.

What to change

1. Replace the CSSPropertyFilter class by the new ElemHideEmulationFilter class.
2. The new filter class needs to have flags that are set based on which extensions are being used in the filter. For now, one flag for CSS properties and one for :has would suffice.
3. The CSS property filters content script needs to be updated so it ignores filters that don't have the CSS properties flag set.

The :has() pseudo-class doesn't actually have to be supported at this point, that's what #3143 is for.

Please note that the Chrome/Firefox specific changes will happen in dedicated issues that will be created once this has landed.

[fhd]

Given how some ElemHideEmulationFilters might actually only use extensions that do not need to be emulated on some browsers (i.e. already supported new CSS features), I think it might be best to do feature detection before deciding whether or not a filter is a plain ElemHideFilter or actually an ElemHideEmulationFilter. That way, the content scripts won't even have to bother with those.

Wladimir, what do you think? (Also in general, I'm reluctant to put much energy into this before it's set to Ready.)

[trev]

Given that :has() isn't supported by any browsers whatsoever this is a purely theoretical discussion at the moment. Of course we could do feature detection, and we've done it in the past. It's merely a temporary solution however, and right now it's not necessary.

[abpbot]

A commit referencing this issue has landed:
​Issue 4394 - Create a filter class for element hiding emulation filters

[Ross]

Tested this change as part of #4658 and it appears to be working okay.

(Leaving ticket open until Chrome changes).

ABP 2.8.2.4246-beta
Firefox 51 / 53 / Beta (54.0b2)
Windows 7