Confirmed - cause found, patch not written yet.

The shop buttons act on mouse-DOWN, so the following mouse-UP lands on whatever sits underneath - the HUD TRADE button behind EXIT - and warps you to the Free Market.

Moving it out of OPEN so the list reflects reality: this one is pinned down to exact files and lines, so what is left is writing and testing the change, not working out what is wrong.

Fixed in 2a75228a.

Cause: one physical click was landing on two different objects. The Cash Shop bound every click target to mouse-DOWN; the HUD bar fires its big buttons on mouse-UP. And the two rectangles overlap: EXIT covers x632-799 / y545-593, and the HUD's TRADE button sits at x630-683 / y564-597. That is a 52x30 patch of EXIT sitting directly on top of TRADE - exactly the spot you described.

So mouse-down hit EXIT (it draws above the HUD, and the topmost object wins), and EXIT's handler closed the shop immediately, destroying the full-screen plate that was blocking the world underneath. The engine re-tests what is under the cursor on every event, so the following mouse-UP found the shop gone and handed itself to TRADE, which warped you to the Free Market. TRADE's own mouse-down never happened; only its release did. Stopping propagation could not have helped, because the two events were never on the same object.

Fix: the shop's click targets are now consistent with the HUD's, so the press and the release land on the same object and the shop cannot vanish between them.

Retest: open the Cash Shop and click EXIT with the cursor right over where TRADE sits.