FvwmEvent: flip prev_focused monitor

When handling focus between screens (which is predicated on windows
receiving focus, rather than the root window), toggle the appropriate
monitors so that the previous focused monitor is referenced correctly.

While here, improve the matching of the previous monitor.

Fixes #604

fvwm/events.c

@@ -2582,11 +2582,13 @@ void HandleFocusIn(const evh_args_t *ea)
 			pfm = monitor_resolve_name(prev_focused_monitor);
 
 			if (fw != NULL && fw->m != pfm) {
-				pfm->is_prev = true;
-				fw->m->is_prev = false;
 				BroadcastName(MX_MONITOR_FOCUS, -1, -1, -1,
 				    fw->m->si->name /* Name of the monitor. */
 			        );
+
+				/* Toggle the state of the previous monitor. */
+				pfm->is_prev = false;
+				fw->m->is_prev = true;
 				if (pfm->virtual_scr.CurrentDesk !=
 				    fw->m->virtual_scr.CurrentDesk)
 					EWMH_SetCurrentDesktop(fw->m);

libs/FScreen.c

@@ -167,8 +167,11 @@ struct monitor *
 monitor_get_prev(void)
 {
 	struct monitor	*m, *mret = NULL;
+	struct monitor  *this = monitor_get_current();
 
 	TAILQ_FOREACH(m, &monitor_q, entry) {
+		if (m == this)
+			continue;
 		if (m->is_prev) {
 			mret = m;
 			break;
@@ -537,6 +540,7 @@ void FScreenInit(Display *dpy)
 		m->Desktops->next = NULL;
 		m->Desktops->desk = 0;
 		m->flags |= (MONITOR_NEW|MONITOR_ENABLED);
+		m->is_prev = false;
 		monitor_scan_edges(m);
 	}
 
@@ -572,6 +576,7 @@ monitor_dump_state(struct monitor *m)
 			"\tDisabled:\t%s\n"
 			"\tIs Primary:\t%s\n"
 			"\tIs Current:\t%s\n"
+			"\tIs Previous:\t%s\n"
 			"\tOutput:\t%d\n"
 			"\tCoords:\t{x: %d, y: %d, w: %d, h: %d}\n"
 			"\tVirtScr: {\n"
@@ -586,6 +591,7 @@ monitor_dump_state(struct monitor *m)
 			(m2->flags & MONITOR_DISABLED) ? "true" : "false",
 			(m2->flags & MONITOR_PRIMARY) ? "yes" : "no",
 			(mcur && m2 == mcur) ? "yes" : "no",
+			m2->is_prev ? "yes" : "no",
 			(int)m2->si->rr_output,
 			m2->si->x, m2->si->y, m2->si->w, m2->si->h,
 			m2->virtual_scr.VxMax, m2->virtual_scr.VyMax,