TL;DR: Drag and drop involving chromium or electron windows has been buggy. wlroots/sway has not implemented _NET_CLIENT_LIST_STACKING, and both the fallback code inside Chrome/Chromium and the way sway has been stacking Xwayland windows have been buggy. A patch has been submitted to Chromium to fix the faulty fallback, the sway bug will be fixed, and wlroots will also support _NET_CLIENT_LIST_STACKING, so by the time the next sway release comes out everything should be ironed out.
TL;DR: Drag and drop involving chromium or electron windows has been buggy. wlroots/sway has not implemented _NET_CLIENT_LIST_STACKING, and both the fallback code inside Chrome/Chromium and the way sway has been stacking Xwayland windows have been buggy. A patch has been submitted to Chromium to fix the faulty fallback, the sway bug will be fixed, and wlroots will also support _NET_CLIENT_LIST_STACKING, so by the time the next sway release comes out everything should be ironed out.