Use correct stairtype for single-layer stair designations

Author: NicksWorld

changelog.txt

@@ -32,6 +32,7 @@ Template for new versions:
 ## New Features
 
 ## Fixes
+- `gui/design`: designating a single-level stair construction now properly follows the selected stair type.
 
 ## Misc Improvements
 

gui/design.lua

@@ -1345,8 +1345,15 @@ function Design:get_designation(point)
     if mode.desig == 'i' then
         local stairs_top_type = self.subviews.stairs_top_subtype:getOptionValue()
         local stairs_bottom_type = self.subviews.stairs_bottom_subtype:getOptionValue()
+        local stairs_only_type = self.subviews.stairs_only_subtype:getOptionValue()
         if point.z == 0 then
-            return stairs_bottom_type == 'auto' and 'u' or stairs_bottom_type
+            if view_bounds.z1 == view_bounds.z2 then
+                -- Single layer staircase
+                return stairs_only_type
+            else
+                -- Bottom of multi-level staircase
+                return stairs_bottom_type == 'auto' and 'u' or stairs_bottom_type
+        end
         elseif view_bounds and point.z == math.abs(view_bounds.z1 - view_bounds.z2) then
             local pos = Point{x=view_bounds.x1, y=view_bounds.y1, z=view_bounds.z1} + point
             local tile_type = dfhack.maps.getTileType(xyz2pos(pos.x, pos.y, pos.z))