Bagin Sorry for the confusion. The latest Spine has a variety of optional settings to make it easier for stylus users to operate, so please check to make sure they are correct first. For example, the setting that allows you to delete a key with a double-click can be found under the Behavior section:

I'm not sure why the hotkeys don't respond unless you press them twice, but you might want to try resetting them to their default settings and see if that works. To restore the hotkeys to their default settings, you can both back up and recreate the hotkey files with the default settings by temporarily moving the hotkeys.txt file in your home folder to another folder and restarting Spine.
(For your information, reinstalling Spine does not reset the files in the user folder. This specification is intended to allow users to immediately use their previously customized settings after reinstalling. Therefore, you must set these settings manually.)

An easy way to open the user folder is to use the Browse button under Backup in the Files section, where you will find the hotkeys.txt file since it is located just above the backup folder:

Bagin I am sorry to hear that checking the settings did not solve the problem. Unfortunately, the issue with the stylus is quite difficult for us to reproduce as it is affected by the settings or specification of the stylus itself, and it may take some time to investigate this issue. Could you please tell us what type of stylus you are using for reference?

We have Intous and Cintiq but haven't seen this problem, and other users haven't reported it -- not to dismiss your issue, just that makes it hard to assist. There are many settings in Windows and the tablet drivers, maybe some combination of settings causes this problem. Are you running any other software that involves keyboard or stylus shortcuts? Anything that might narrow it down would help.

Can you please post your entire spine.log? That will tell us what operating system are you using and may provide other clues.

Does double click work in other areas? When "double click shortcuts" are enabled, double clicking in the viewport will deselect items in the viewport. Does that work? If you double click an item in the tree, does it open the rename dialog?

animation keys on dopesheet are either not deleted at all, or are deleted 10-15 times

This sounds odd. Can you explain exactly what you mean that the key is deleted 10-15 times?

Thanks for the additional information.

Those people in the chat should report their issues so we can fix them! Don't suffer in silence! We want to fix issues and appreciate reports.

3.8 is very old and different in many ways, so that it works there doesn't really help us narrow it down.

Since double click works everywhere else, that likely eliminates any stylus drivers or interference from other software. Since double click works sometimes, it's likely something to do with how you are clicking and how we detect and apply the clicking.

After some digging it seems the problem is that moving the mouse at all (1 pixel) when the mouse button is down will start a drag. If a drag has started, then double click won't delete the key. You can see the drag has started when the key under the mouse is cyan rather than white.

We'll fix this in 4.2.39 so the drag doesn't start until the mouse moves a significant difference, like most drags in Spine. Sorry this caused you so much trouble, but thanks for letting us know about the problem!

Sorry, you're right, it didn't make it into 4.2.39. Should be in 4.2.40 though! We'll try to release that soon.

Great, thanks for confirming the fix! Sorry it took so long.