I'm seeing a distinct cosmetic problem with the general cursor appearance in recent 4.2.0 pre-alpha appimages.
The problem does not happen with the 4.1.7 original appimage or the 16 March 4.1.7.101 stable nightly build appimage or the 21 Feb 4.2.0-pre-alpha (git 7dc0c43).
It does happen with the 12 March 4.2.0-pre-alpha (git 3805574) and the 16 March 4.2.0-pre-alpha (git 3e31645) appimages.

All the appimages use Qt 5.10.0 running on Linux Debian 9, Kernel Version 4.9.0-8-amd64 according to their bug report information.

The problem does not happen with the mouse, it only happens with the tablet/pen as follows:

When the cursor is moved from the canvas to the Toolbox or any docker, it passes over the docker area boundary and becomes the double headed left-right arrow (or up/down arrow for dockers below the canvas) and it stays that way until you use it to click something.
The main 'UI experience' effect of this is that you select a tool from the Toolbox and move over to the canvas but the cursor does not change to the icon for the tool. If you touch the canvas, the brush doesn't work until you lift it off and touch again. The correct icon is shown after you touch the canvas for the first time. It then works, until you move it off-canvas again.

When it's a double headed arrow, it does not repond properly to the Space or Cntrl+Space or Cntrl keys (it becomes a pointer arrow) except after you've used them once (yes, it's complicated).

The tablet tester utility only shows stylus press and move actions and stylus release actions for these 'problem' appimages. For the 'non-problem' appimages, the tablet tester shows mouse press/moves along with the stylus actions when the stylus is used.

It's part of a set of qt 5.12 bugs we're trying to fix. In particular we need to use Qt's own XCB input, which the appimages may have switched to to prevent crashes despite their deps still using qt 5.10

Related bugs:
https://bugs.kde.org/show_bug.cgi?id=390642
https://bugs.kde.org/show_bug.cgi?id=404321
https://bugs.kde.org/show_bug.cgi?id=399664

This whole thing started because the scroll stopped working when qt 5.12 was used to build with.
https://bugs.kde.org/show_bug.cgi?id=399379

This is separate from another set of problems that got into 5.12, namely that qt decided to implement their own version of winink support and that conflicts with krita's:

https://bugs.kde.org/show_bug.cgi?id=405494

As well as stuff that may be related to the new graphics acceleration stuff:
https://bugs.kde.org/show_bug.cgi?id=405540

tl;dr: We're fighting with an updated dependency, and thus everything nightly will be chaotic for a while.