When using it with a tablet, I consistly hit one of the pen buttons both by muscle memory and by mistake. This pastes the clipboard and changes my tool to the text tool, and is generally annoying.
I understand why one might think it’s reasonable, but this behavior has little use and is a consistent interruption.
Could be the system-wide setting being active?
System Settings → Workspace Behavior → General Behavior → Middle Click: Uncheck “Paste selected text”
I want it in the rest of the OS, it just has no place on art software.
played a bit with this and there is already a behavior built-in that seems to avoid this issue of pasting text into the app with the middle mouse click (at least on my version of kolourpaint from 3 years ago).
if i’ve highlighted some text then the buffer is loaded and middle click will, as you say, switch you to the text tool and paste the text onto the canvas.
however, if you go to the clipboard and select an image or copy and image to the clipboard so that it is loaded into the ctrl-V paste buffer, then middle click does NOT result in pasting any text (it doesn’t do anything at all).
if your version of kolourpaint does not display this behavior, then i would suggest filing a regression bug report at bugs.kde.org