Properties Window:

*The narrowui value was hard coded in all ui scripts, made an user preferences option. Basically this value determines on which area width, it should switch between dual/single column layout. 

ToDo: The Changes only take effect when reloading scripts/restarting Blender (after saving as default). Will maybe add the "Reload Scripts" operator next to the button in the future. 

* Small fix for Fluid Add Button, when in single column mode. Didn't expand like the other "Add" Buttons.

release/scripts/ui/properties_physics_fluid.py

@@ -19,7 +19,7 @@
 # <pep8 compliant>
 import bpy
 
-narrowui = 180
+narrowui = bpy.context.user_preferences.view.properties_width_check
 
 
 class PhysicButtonsPanel(bpy.types.Panel):
@@ -42,7 +42,7 @@ class PHYSICS_PT_fluid(PhysicButtonsPanel):
         md = context.fluid
         wide_ui = context.region.width > narrowui
 
-        split = layout.split(percentage=0.5)
+        split = layout.split()
 
         if md:
             # remove modifier + settings