Reordering a bit.

Project/scripts/button_plusminus_shortcut/button_plusminus_shortcut.gml

@@ -0,0 +1,25 @@
+///button_plusminus_shortcut(button, pressed?)
+
+var button = argument0;
+var pressed = argument1;
+
+if (instance_exists(obj_lock))
+{
+    switch(button)
+    {
+        case but.plus:  var hold = obj_lock.plus;  break;
+        case but.minus: var hold = obj_lock.minus; break;
+        
+        default:
+            show_debug_message("ERROR: Wrong parameter assignment.");
+            return;
+        break;
+    }
+    
+    hold.hover = true;
+    hold.pressed = pressed;
+    
+    if (pressed)
+        with (hold)
+            button_do(type);
+}

Project/scripts/button_plusminus_shortcut/button_plusminus_shortcut.yy

@@ -0,0 +1,8 @@
+{
+    "id": "b0a1886d-571c-9ed1-098f-65abb5bf18f6",
+    "modelName": "GMScript",
+    "mvc": "1.0",
+    "name": "button_plusminus_shortcut",
+    "IsCompatibility": false,
+    "IsDnD": false
+}