1. Two or more application windows are grouped into one icon in taskbar.
2. When you hover over this icon - a series of thumbnails of the grouped windows appears.
3. If you move the cursor diagonally to window thumbnail, and even slightly (literally a pixel) touches the neighboring task-icon of another application , then the thumbnails of another application will pop out.
So I have to move the cursor like chess knight (up then side) to select apps window from previews.
This is not the same https://bugs.kde.org/show_bug.cgi?id=375671
*** This bug has been marked as a duplicate of bug 358930 ***