KDE Taskbar Drag-and-Drop
A distracting KDE bug is clicking on a taskbar icon with the mouse sometimes invokes drag-and-drop behavior rather than changing focus to the respective window. The “drag” seems triggered by moving the mouse cursor upward from the taskbar into the destination window, such as when wanting to continue working in the Kate text editor. Tinkering with the mouse
Drag start time=1000 msecs and
Drag start distance=20 pixels is ongoing, but might be helping. Task switching with the keyboard
Alt+Tab shortcut is another option.
The problem has been reported. The bug report discussion seems to indicate the problem might be patched in KDE 5.26.
Hopefully the bug is resolved and Slackware 15.1 with KDE 5.27 will be released sooner rather than later.