Created attachment 174301 [details]
Simple program reproducing the issue

Steps to reproduce:
1. Compile the attached program using the following command:
gcc `pkg-config --cflags --libs gtk+-2.0` gtk_calendar_test.c
2. Run the program: ./a.out
3. Press the button to emerge a calendar

Expected result:
The calendar responds to mouse clicks.

Actual result:
The calendar does not respond to mouse clicks.

Notes:
1. This issue is not reproducible with GTK2:
gcc `pkg-config --cflags --libs gtk+-2.0` gtk_calendar_test.c
2. This issue does not seem to be related to the type of the box accepting the calendar (at least it's also reproducible when attaching calendar to a table).