expand: add desk, pagex, pagey variables

This adds a few more variables which exposes more internal state for use
in functions and modules.

doc/fvwm3/fvwm3.adoc

@@ -1661,7 +1661,7 @@ $[pointer.screen]::
 +
 This command is deprecated; use $[monitor.current] instead.
 
-$[monitor.<n>.x], $[monitor.<n>.y], $[monitor.<n>.width], $[monitor.<n>.height], $[monitor.primary], $[monitor.current], $[monitor.output], $[monitor.count]::
+$[monitor.<n>.x], $[monitor.<n>.y], $[monitor.<n>.width], $[monitor.<n>.height], $[monitor.<n>.desk], $[monitor.<n>.pagex], $[monitor.<n>.pagey] $[monitor.primary], $[monitor.current], $[monitor.output], $[monitor.count]::
 	Returns information about the selected monitor. These can be nested, for
 	example: $[monitor.$[monitor.primary].width]
 +
@@ -1676,6 +1676,12 @@ monitor which has the mouse pointer.
 +
 "count" returns the number of active monitors.
 +
+"desk" returns the current desk displayed on the referenced monitor.
++
+"pagex" returns the X page on the referenced monitor.
++
+"pagey" returns the Y page of the referenced monitor.
++
 "primary" is the name of the output set as primary via xrandr(1).
 
 $[screen]::

fvwm/expand.c

@@ -571,7 +571,27 @@ static signed int expand_vars_extended(
 
 			if (strcmp(rest, "output") == 0) {
 				is_numeric = True;
-				val = (int)m->si->rr_output;
+				val = (int)mon->si->rr_output;
+				goto GOT_STRING;
+			}
+
+			if (strcmp(rest, "desk") == 0) {
+				is_numeric = True;
+				val = mon->virtual_scr.CurrentDesk;
+				goto GOT_STRING;
+			}
+
+			if (strcmp(rest, "pagex") == 0) {
+				is_numeric = True;
+				val = (int)(mon->virtual_scr.Vx /
+					monitor_get_all_widths());
+				goto GOT_STRING;
+			}
+
+			if (strcmp(rest, "pagey") == 0) {
+				is_numeric = True;
+				val = (int)(mon->virtual_scr.Vy /
+					monitor_get_all_heights());
 				goto GOT_STRING;
 			}
 		}