Author: Eric Pruitt, https://github.com/jameseric/
Description: Switch to most recently, previously focused window that is
currently visible.

--- a/dwm.c	2012-12-09 23:48:53.000000000 -0600
+++ b/dwm.c	2012-12-09 23:48:53.000000000 -0600
@@ -30,6 +30,7 @@
 #include <unistd.h>
 #include <sys/types.h>
 #include <sys/wait.h>
+#include <sys/time.h>
 #include <X11/cursorfont.h>
 #include <X11/keysym.h>
 #include <X11/Xatom.h>
@@ -95,6 +96,7 @@
 	Client *snext;
 	Monitor *mon;
 	Window win;
+	struct timeval ft;
 };
 
 typedef struct {
@@ -224,6 +226,7 @@
 static void showhide(Client *c);
 static void sigchld(int unused);
 static void spawn(const Arg *arg);
+static void lastclient();
 static void tag(const Arg *arg);
 static void tagmon(const Arg *arg);
 static int textnw(const char *text, unsigned int len);
@@ -865,6 +868,21 @@
 }
 
 void
+lastclient() {
+	Client *c, *o;
+	for (o = selmon->sel, c = selmon->clients; c; c = c->next) {
+		if (ISVISIBLE(c) && c != selmon->sel && (o == selmon->sel ||
+		  ((double)c->ft.tv_sec + c->ft.tv_usec * 0.000001) >
+		  ((double)o->ft.tv_sec + o->ft.tv_usec * 0.000001))) {
+			o = c;
+		}
+	}
+
+	if(o != selmon->sel)
+		focus(o);
+}
+
+void
 focusin(XEvent *e) { /* there are some broken focus acquiring clients */
 	XFocusChangeEvent *ev = &e->xfocus;
 
@@ -1520,6 +1538,8 @@
 
 void
 setfocus(Client *c) {
+	if(gettimeofday(&c->ft, NULL))
+		perror("gettimeofday(&c->ft, NULL) failed");
 	if(!c->neverfocus)
 		XSetInputFocus(dpy, c->win, RevertToPointerRoot, CurrentTime);
 	sendevent(c, wmatom[WMTakeFocus]);