Use the brightness slider to change the brightness via an alpha layer (and stop using the backlight, if you have one). Note that this extension will keep running on the lock screen, as you'd also want the brightness setting to apply to the lock screen as well. Please comment if this gives you any trouble.
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I have approved v3 of your extension, however this version does not setup/cleanup in `enable()` and `disable()`. Unless you'd like to resubmit with those changes, I'll leave this for another reviewer to reject or accept.
Version 3 doesn't clean-up in disable() either, this is an intended behavior. I understand the rationale behind the rule of cleaning up on disable(). If you look closely, the extension's behavior is to skip disabling when disable() is called when the lock screen is activating. This is intentional as the user may not want the lock screen to be on full-brightness. In all other cases besides the lock screen activating, disable() effectively disables the extensions, disconnects all signals etc. There's unfortunately or not no concept of "trusted extension" that would be allowed to keep running on the lock screen. This extension (all versions I've uploaded from v1 to v4) only partially disables the extension when the lock screen is activating: disable() is called, but _disable() isn't. Would you consider a compromise where the extension would be disabled normally on the lock screen (disable() would be always effective and would call _disable()) , but which could be toggled by a preference that would enable the current behavior (extension keeps running on the lock screen, disable() doesn't call _disable() when called upon entering the lock screen).
Disregard, my mistake.
Sorry, to clarify, I missed that enable()/disable() had been moved to the extension object. Since this replaces an option already available on the lock-screen I understand your approach. Perhaps a note to users that this is the case would be appropriate.
Thanks for your constructive comment I've added this paragraph to the description: Note that this extension will keep running on the lock screen, as you'd also want the brightness setting to apply to the lock screen as well. Please comment if this gives you any trouble.