Drag a window list widget onto the desktop, stretch it a bit and there's a pin icon on the top right corner. Clicking it takes no effect. This should only appear when the window list is a popup list.
Can you grab a screenshot of how far you're stretching it open? I'm not seeing the pin except in the expanded (clicked) view.
Created attachment 111057 [details] screenshot
Can reproduce this issue in: Plasma: 5.12.2 Apps: 17.12.2 Frameworks: 5.43.0 Qt: 5.10.1 Kernel: 4.14.22-1-MANJARO OS: Netrunner Rolling
Git commit eaa5b4364f8bd59869a947f1bcf3edd6a06d01c0 by Kai Uwe Broulik. Committed on 16/04/2018 at 14:41. Pushed by broulik into branch 'Plasma/5.12'. [Window List] Show pin icon only when in panel FIXED-IN: 5.12.5 Differential Revision: https://phabricator.kde.org/D12146 M +2 -0 applets/window-list/contents/ui/main.qml https://commits.kde.org/plasma-desktop/eaa5b4364f8bd59869a947f1bcf3edd6a06d01c0