I am testing Krita 4.2 and 4.2.1, and I experience the following problems, with the instant preview activated:

1.- I can change the size brush, but the slider of the upper toolbar does not move. By not exceeding the preset minimum size, the instant preview does not work. The problem disappears changing the brush and going back to the first one. It happens for some brushes only. Here is someone who tells the same, with an explanatory video:

https://www.reddit.com/r/krita/comments ... al_issues/

2.- The smooth zoom, it works badly, it does not go fluid, it gets stuck in some points. It happens in large images, with many layers, but in the same images, the zoom goes very fast in 4.1.7. If I deactivate the quick view, everything is fine.

Intel Core i7-7700 3.60 Ghz CPU, 8 Gigas Ram, integrated graphics. OS Manjaro KDE. Wacom Tablet PTH-851.

For now, I should stay with 4.1.7

What do you mean by "smooth zoom"? Is it like two-finger zoom-in/-out gestures on touchscreens? If you are referring to that, I should say I have that same problem, and it is probably a bug. I suggest making a bug report on bugs.kde.org. However it could possibly be a Qt bug.