Respect decoration hints

Make dwm respect _MOTIF_WM_HINTS property.  Applications use this
property to notify window managers to not draw window decorations.

Not respecting this property leads to issues with applications that draw
their own borders, like chromium (with "Use system title bar and
borders" turned off) and vlc in fullscreen mode.

config.def.h

@@ -40,6 +40,7 @@ static const Rule rules[] = {
 static const float mfact     = 0.55; /* factor of master area size [0.05..0.95] */
 static const int nmaster     = 1;    /* number of clients in master area */
 static const int resizehints = 1;    /* 1 means respect size hints in tiled resizals */
+static const int decorhints  = 1;    /* 1 means respect decoration hints */
 
 static const Layout layouts[] = {
 	/* symbol     arrange function */

dwm.c

@@ -57,6 +57,13 @@
 #define TAGMASK                 ((1 << LENGTH(tags)) - 1)
 #define TEXTW(X)                (drw_fontset_getwidth(drw, (X)) + lrpad)
 
+#define MWM_HINTS_FLAGS_FIELD       0
+#define MWM_HINTS_DECORATIONS_FIELD 2
+#define MWM_HINTS_DECORATIONS       (1 << 1)
+#define MWM_DECOR_ALL               (1 << 0)
+#define MWM_DECOR_BORDER            (1 << 1)
+#define MWM_DECOR_TITLE             (1 << 3)
+
 /* enums */
 enum { CurNormal, CurResize, CurMove, CurLast }; /* cursor */
 enum { SchemeNorm, SchemeSel }; /* color schemes */
@@ -221,6 +228,7 @@ static void updatebarpos(Monitor *m);
 static void updatebars(void);
 static void updateclientlist(void);
 static int updategeom(void);
+static void updatemotifhints(Client *c);
 static void updatenumlockmask(void);
 static void updatesizehints(Client *c);
 static void updatestatus(void);
@@ -261,7 +269,7 @@ static void (*handler[LASTEvent]) (XEvent *) = {
 	[PropertyNotify] = propertynotify,
 	[UnmapNotify] = unmapnotify
 };
-static Atom wmatom[WMLast], netatom[NetLast];
+static Atom wmatom[WMLast], netatom[NetLast], motifatom;
 static int running = 1;
 static Cur *cursor[CurLast];
 static Clr **scheme;
@@ -1086,6 +1094,7 @@ manage(Window w, XWindowAttributes *wa)
 	updatewindowtype(c);
 	updatesizehints(c);
 	updatewmhints(c);
+	updatemotifhints(c);
 	XSelectInput(dpy, w, EnterWindowMask|FocusChangeMask|PropertyChangeMask|StructureNotifyMask);
 	grabbuttons(c, 0);
 	if (!c->isfloating)
@@ -1272,6 +1281,8 @@ propertynotify(XEvent *e)
 		}
 		if (ev->atom == netatom[NetWMWindowType])
 			updatewindowtype(c);
+		if (ev->atom == motifatom)
+			updatemotifhints(c);
 	}
 }
 
@@ -1602,6 +1613,7 @@ setup(void)
 	netatom[NetWMWindowType] = XInternAtom(dpy, "_NET_WM_WINDOW_TYPE", False);
 	netatom[NetWMWindowTypeDialog] = XInternAtom(dpy, "_NET_WM_WINDOW_TYPE_DIALOG", False);
 	netatom[NetClientList] = XInternAtom(dpy, "_NET_CLIENT_LIST", False);
+	motifatom = XInternAtom(dpy, "_MOTIF_WM_HINTS", False);
 	/* init cursors */
 	cursor[CurNormal] = drw_cur_create(drw, XC_left_ptr);
 	cursor[CurResize] = drw_cur_create(drw, XC_sizing);
@@ -1983,6 +1995,39 @@ updategeom(void)
 	return dirty;
 }
 
+void
+updatemotifhints(Client *c)
+{
+	Atom real;
+	int format;
+	unsigned char *p = NULL;
+	unsigned long n, extra;
+	unsigned long *motif;
+	int width, height;
+
+	if (!decorhints)
+		return;
+
+	if (XGetWindowProperty(dpy, c->win, motifatom, 0L, 5L, False, motifatom,
+	                       &real, &format, &n, &extra, &p) == Success && p != NULL) {
+		motif = (unsigned long*)p;
+		if (motif[MWM_HINTS_FLAGS_FIELD] & MWM_HINTS_DECORATIONS) {
+			width = WIDTH(c);
+			height = HEIGHT(c);
+
+			if (motif[MWM_HINTS_DECORATIONS_FIELD] & MWM_DECOR_ALL ||
+			    motif[MWM_HINTS_DECORATIONS_FIELD] & MWM_DECOR_BORDER ||
+			    motif[MWM_HINTS_DECORATIONS_FIELD] & MWM_DECOR_TITLE)
+				c->bw = c->oldbw = borderpx;
+			else
+				c->bw = c->oldbw = 0;
+
+			resize(c, c->x, c->y, width - (2*c->bw), height - (2*c->bw), 0);
+		}
+		XFree(p);
+	}
+}
+
 void
 updatenumlockmask(void)
 {