I have this weird behavior, that when I pick transparent colors from current layer (ctrl+alt), picked color's transparency affects the texture on brushes - the more transparent color is picked, brush's texture has higher contrast. It goes back to normal when moving the color wheel, even just little.
Is it some feature?

I can't that that rings a bell. I probably would need to see a screen recording of it.

I've just tried this with yesterday's Nightly appimage and it does seem to happen.

Cntrl Pick is influenced by the opacity value of the current layer provided that it is over an opaque background layer. If the current layer is not over an opaque background then only the colour value is picked up. Either way, it's the 'visually apparent' colour value that's picked up and applied with 100% opacity as painted. (By that I mean that red:25% opacity will be picked up and painted as pink:100% opacity if over an opaque background. If not over an opaque background, then red:25% will be painted as red:100%)

Cntrl+Alt Pick will not pick up opacity information at all and paints red:25% as red:100% in both cases.

The pick up result, as shown in the Specific Colour Selector, is opaque for a Ctrl Pick and shows transparency for a Ctrl+Alt Pick (assuming the picked colour has transparency) if there is an opaque layer underneath. With no opaque background layer, they both give a Specific Clour Selector result that shows the transparency but they both give paint at the original colour, e.g red:25% as red:100% as noted in the previous two paragraphs.

I've no idea what it's supposed to do.

Here's the video of the behavior: https://drive.google.com/open?id=1lPJwT ... fQPLCYjdjK