Turn off screen and backlight on state change only.

slock.c

@@ -734,7 +734,8 @@ readpw(struct arena *arena,
 {
     XRRScreenChangeNotifyEvent *rre;
     char buf[32], passwd[256], *inputhash;
-    int num, screen, running, failure, locked, oldc, i, passing, dson, tfd, xfd;
+    int num, screen, running, failure, locked, oldc, i, passing,
+        dson, dson_saved, tfd, xfd;
     unsigned int len, color;
     struct timeval tv;
     KeySym ksym;
@@ -751,7 +752,9 @@ readpw(struct arena *arena,
     failure = 0;
     locked = 1;
     oldc = INIT;
-    dson = 1;
+
+    dson  = 1;
+    dson_saved = 1;
 
     xfd = ConnectionNumber(dpy);
     tfd = timerfd_create(CLOCK_REALTIME, 0);
@@ -905,13 +908,16 @@ readpw(struct arena *arena,
             }
         }
 
-        if(dson) {
-            reset_keyboard_backlight(bl_state);
-            set_screen_on(dpy);
-        } else {
-            set_keyboard_backlight_off(bl_state);
-            set_screen_off(dpy);
+        if(dson_saved != dson) {
+            if(dson) {
+                reset_keyboard_backlight(bl_state);
+                set_screen_on(dpy);
+            } else {
+                set_keyboard_backlight_off(bl_state);
+                set_screen_off(dpy);
+            }
         }
+        dson_saved = dson; 
 
         render_screen(arena,
                       dpy, locks,