add patch/function 'goback'

config.def.h

@@ -85,6 +85,7 @@ static const Key keys[] = {
 	{ MODKEY,                       XK_l,      setmfact,       {.f = +0.05} },
 	{ MODKEY,                       XK_Return, zoom,           {0} },
 	{ MODKEY,                       XK_Tab,    view,           {0} },
+	{ MODKEY,                       XK_g,      goback,         {0} },
 	{ MODKEY|ShiftMask,             XK_c,      killclient,     {0} },
 	{ MODKEY,                       XK_t,      setlayout,      {.v = &layouts[0]} },
 	{ MODKEY,                       XK_f,      setlayout,      {.v = &layouts[1]} },

dwm.c

@@ -180,6 +180,7 @@ static Atom getatomprop(Client *c, Atom prop);
 static int getrootptr(int *x, int *y);
 static long getstate(Window w);
 static int gettextprop(Window w, Atom atom, char *text, unsigned int size);
+static void goback(const Arg *arg);
 static void grabbuttons(Client *c, int focused);
 static void grabkeys(void);
 static void hide(const Arg *arg);
@@ -287,7 +288,7 @@ static Cur *cursor[CurLast];
 static Clr **scheme, **schemedark, **schemelight;
 static Display *dpy;
 static Drw *drw;
-static Monitor *mons, *selmon;
+static Monitor *mons, *selmon, *prevmon;
 static Window root, wmcheckwin;
 static const char **dmenucmd;
 static int colormodechanged;
@@ -452,6 +453,7 @@ buttonpress(XEvent *e)
 	/* focus monitor if necessary */
 	if ((m = wintomon(ev->window)) && m != selmon) {
 		unfocus(selmon->sel, 1);
+		prevmon = selmon;
 		selmon = m;
 		focus(NULL);
 	}
@@ -840,6 +842,7 @@ enternotify(XEvent *e)
 	m = c ? c->mon : wintomon(ev->window);
 	if (m != selmon) {
 		unfocus(selmon->sel, 1);
+		prevmon = selmon;
 		selmon = m;
 	} else if (!c || c == selmon->sel)
 		return;
@@ -872,8 +875,10 @@ focus(Client *c)
 		}
 	}
 	if (c) {
-		if (c->mon != selmon)
+		if (c->mon != selmon) {
+			prevmon = selmon;
 			selmon = c->mon;
+		}
 		if (c->isurgent)
 			seturgent(c, 0);
 		detachstack(c);
@@ -909,6 +914,7 @@ focusmon(const Arg *arg)
 	if ((m = dirtomon(arg->i)) == selmon)
 		return;
 	unfocus(selmon->sel, 0);
+	prevmon = selmon;
 	selmon = m;
 	focus(NULL);
 	warp(selmon->sel);
@@ -1031,6 +1037,21 @@ gettextprop(Window w, Atom atom, char *text, unsigned int size)
 	return 1;
 }
 
+void
+goback(const Arg *arg)
+{
+	if (prevmon == NULL) {
+		Arg a = {0};
+		view(&a);
+	} else if (prevmon != selmon) {
+		unfocus(selmon->sel, 0);
+		Monitor *p = selmon;
+		selmon = prevmon;
+		focus(NULL);
+		prevmon = p;
+	}
+}
+
 void
 grabbuttons(Client *c, int focused)
 {
@@ -1291,6 +1312,8 @@ motionnotify(XEvent *e)
 		return;
 	if ((m = recttomon(ev->x_root, ev->y_root, 1, 1)) != mon && mon) {
 		unfocus(selmon->sel, 1);
+		if (m != selmon)
+			prevmon = selmon;
 		selmon = m;
 		focus(NULL);
 	}
@@ -1352,6 +1375,7 @@ movemouse(const Arg *arg)
 	XUngrabPointer(dpy, CurrentTime);
 	if ((m = recttomon(c->x, c->y, c->w, c->h)) != selmon) {
 		sendmon(c, m);
+		prevmon = selmon;
 		selmon = m;
 		focus(NULL);
 	}
@@ -1543,6 +1567,7 @@ resizemouse(const Arg *arg)
 	while (XCheckMaskEvent(dpy, EnterWindowMask, &ev));
 	if ((m = recttomon(c->x, c->y, c->w, c->h)) != selmon) {
 		sendmon(c, m);
+		prevmon = selmon;
 		selmon = m;
 		focus(NULL);
 	}
@@ -2081,6 +2106,7 @@ toggleview(const Arg *arg)
 	unsigned int newtagset = selmon->tagset[selmon->seltags] ^ (arg->ui & TAGMASK);
 
 	if (newtagset) {
+		prevmon = NULL;
 		selmon->tagset[selmon->seltags] = newtagset;
 		focus(NULL);
 		arrange(selmon);
@@ -2409,6 +2435,7 @@ view(const Arg *arg)
 {
 	if ((arg->ui & TAGMASK) == selmon->tagset[selmon->seltags])
 		return;
+	prevmon = NULL;
 	selmon->seltags ^= 1; /* toggle sel tagset */
 	if (arg->ui & TAGMASK)
 		selmon->tagset[selmon->seltags] = arg->ui & TAGMASK;