Increasing either 'Lightness' or 'Brightness' above zero or 𝒅𝒆creasing 'Contrast' 𝑏𝑒𝑙𝑜𝑤 zero reduces transparency and starts turning even fully-transparent pixels opaque

[NM64]

Really simple, so I don't think I need to record a video (but let me know if you do want one!), and it occurred with the flatpak of Pinta 2.1.1 on both the live ISO of Linux Mint 21.3 Cinnamon and an installed copy of Linux Mint 20.3 Xfce.

But as for the issue, this is easily verified by just making a new layer (which always defaults to being fully transparent) and then deleting all other layers, leaving you with only that newly-created fully transparent layer.

From there, I've found that any of the following three things will result in transparency is turning more and more opaque:

"Adjustments" ▶ "Hue/Saturation" — set 'Lightness' to a value greater than zero

"Adjustments" ▶ "Brightness/Contrast" — set 'Brightness' to a value greater than zero

"Adjustments" ▶ "Brightness/Contrast" — set 'Contrast' to a value less than zero

The thing is though, doing the opposite does not do this. That is, your fully transparent layer will remain fully transparent if you set 'Lightness' to less than zero, set 'Brightness' to less than zero, or set 'Contrast' to greater than zero.

[Cameron White (cameronwhite91)]

This is probably the cause of the behaviour seen in bug 2051431