Yes, people still play 1.4.53...
When going from "nothing" to something (constructor et al.), clicking "cancel" does not revert the build queue to "nothing." It chooses whatever was highlighted.
Cancel functions properly if there's something in the queue already. If I already have transports being built and momentarily decide to highlight defenders but click cancel, that works fine.
Recap: "cancel" doesn't work properly when there's nothing in the queue.
