1759117 - `runAt` in `scripting.RegisteredContentScript` should not affect CSS injection

Status: RESOLVED WONTFIX

(WebExtensions :: General, enhancement, P2)

Description

[William Durand [:willdurand]]

runAt should only be used for js files and not css ones. Indeed, CSS should unconditionally be applied at document_start, cf. https://bugzilla.mozilla.org/show_bug.cgi?id=1755978#c1.

This isn't exactly trivial, which is why this bug exists, cf. https://bugzilla.mozilla.org/show_bug.cgi?id=1755976#c2.

Comment 1

[Rob Wu [:robwu]]

Here is a sketch of how the unit tests can look be implemented: https://phabricator.services.mozilla.com/D140729?id=555542#inline-775448

Comment 2

[William Durand [:willdurand]]

Given this was already the behavior for MV2 extensions in Firefox and a workaround exists (i.e. register CSS and JS separately to control the runAt value for each), we decided to wontfix this bug. We'll make sure that the documentation mentions the difference between Chromium browsers and FF.