Apparently, the workbench misses a refresh of the backdrop layer after the front window moves away from the clock.
No, in that area with the "trash" there is the ~non-user-visible fake-transparency LimpidClock window, not wb backdrop window. It maybe tries to auto-refresh (close/reopen
window?) the whats-behind-snapshot (initially done by opening window with LAYERS_NO_BACKFILL) at the wrong time.
Problem number 2 is that intuition does not support full-size window movements. IOWs, whatever enables it has to interact with intution in a way that intuition did not natively support. This typically requires working on some internal structures of intuition which may fire back. In particular, the traditional window movement happens in the intuition input handler, and dependencies between this and the intuition and layers semaphores are somewhat fragile. So this is unfortunately also calling for problems.
Since decades the way this was done is to detect click into titlebar in input handler or commodity and then simply move window with a normal Intuition function like MoveWindow() (best in a helper task, not in input.device task context directly to give app tasks time to refresh their simple refresh windows in between while window is dragged). No internals of Intuition touched.