Created attachment 287470 [details]
gtk2 vs gtk3

Below is a bit of sample code to show the issue. 

In gtk3, the days shift to the bottom. The first two row of days (31 1 2 3 ...) can be selected, but anything below cannot. 

Tested on Fedora Linux 20. GtkVersion 3.10.9 



#include <stdio.h>
#include <stdlib.h>
#include <gtk/gtk.h>

int main( int   argc,
          char *argv[] )
{
    //------------------ INIT
	GtkWidget *window;
	GtkWidget *vbox;
	GtkCalendar *calendar;

    gtk_init (&argc, &argv);
    window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    gtk_window_set_title (GTK_WINDOW (window), "GtkCalendar GTK3");
    gtk_window_set_default_size(window, 300, 350);
    gtk_container_set_border_width (GTK_CONTAINER (window), 5);

    //--- Vertical Layout:
    vbox = gtk_vbox_new (FALSE, 10);
    gtk_container_add (GTK_CONTAINER (window), vbox);

    //------------------ ADD ELEMENTS
    calendar=gtk_calendar_new ();
    gtk_container_add (GTK_CONTAINER (vbox), calendar);

    //------------------ POST LOOP
    gtk_widget_show_all (window);
    gtk_main ();

    return 0;
}

Once bug is fixed, could someone please reply in:
https://bugs.eclipse.org/bugs/show_bug.cgi?id=445539

(SWT DateTime is affected by this). 

Thank you

If you want the calendar to stay at the top, just set the valign:

gtk_widget_set_valign (calendar, GTK_ALIGN_START);

reopening; we should still try to improve the rendering with valign = FILL, and possibly there is some problem with clicking on days too

Created attachment 287524 [details]
Screen shot with annotation outlining selection issue

thank you for your comments, 
for clarification, attached is a screen shot with annotation showing the selection issue

Created attachment 287525 [details]
issue in grid layout

I read that gtk_vbox is deprecated as of gtk3.2:
https://developer.gnome.org/gtk3/stable/GtkVBox.html#gtk-vbox-new
It suggests to switch over to grid instead.

If of use, I wrote a bit of code with 'grid' layout where the issue occurs also. (attached).

Created attachment 287526 [details]
Screenshot with annotation for gtk_grid_calendar.c