Update.
When I'm calling hide() and show() for the main window (after the image viewer window becomes active), I get those errors at console output:

X Error: BadWindow (invalid Window parameter) 3
Major opcode: 20 (X_GetProperty)
Resource id: 0x16055ff
X Error: BadWindow (invalid Window parameter) 3
Major opcode: 15 (X_QueryTree)
Resource id: 0x16055ff
X Error: BadWindow (invalid Window parameter) 3
Major opcode: 40 (X_TranslateCoords)
Resource id: 0x16055ff

So the problem is in the X-system?