placement: Fix margin support

This was only partially implemented. The margins were substracted from
the area too early in the pipeline. Now, they are added when getting
the size and substracted when setting it. This way, the margins will
"survive" when a placement function set an absolute value in one of
the field. Previously, this caused one (or more) of the margins to
be lost.

lib/awful/placement.lua

@@ -137,6 +137,7 @@ local function compose(...)
         local pretend_real = args.pretend
 
         args.pretend = true
+        args.attach  = false
 
         for k, f in ipairs(queue) do
             if k == #queue then
@@ -259,29 +260,51 @@ local function store_geometry(d, reqtype)
     data[d][reqtype].screen = d.screen
 end
 
---- Apply some modifications before applying the new geometry.
--- @tparam table new_geo The new geometry
--- @tparam table args The common arguments
--- @treturn table|nil The new geometry
-local function fix_new_geometry(new_geo, args)
-    if args.pretend or not new_geo then return nil end
+--- Get the margins and offset
+-- @tparam table args The arguments
+-- @treturn table The margins
+-- @treturn table The offsets
+local function get_decoration(args)
+    local offset = args.offset
 
-    local offset = args.offset or {}
-
-    if type(offset) == "number" then
-        offset = {
+    -- Offset are "blind" values added to the output
+    offset = type(offset) == "number" and {
         x      = offset,
         y      = offset,
         width  = offset,
         height = offset,
+    } or args.offset or {}
+
+    -- Margins are distances on each side to substract from the area`
+    local m = type(args.margins) == "table" and args.margins or {
+        left = args.margins or 0 , right  = args.margins or 0,
+        top  = args.margins or 0 , bottom = args.margins or 0
     }
+
+    return m, offset
 end
 
+--- Apply some modifications before applying the new geometry.
+-- @tparam table new_geo The new geometry
+-- @tparam table args The common arguments
+-- @tparam boolean force Always ajust the geometry, even in pretent mode. This
+--  should only be used when returning the final geometry as it would otherwise
+--  mess the pipeline.
+-- @treturn table|nil The new geometry
+local function fix_new_geometry(new_geo, args, force)
+    if (args.pretend and not force) or not new_geo then return nil end
+
+    local m, offset = get_decoration(args)
+
     return {
-        x      = new_geo.x      and (new_geo.x      + (offset.x      or 0)),
-        y      = new_geo.y      and (new_geo.y      + (offset.y      or 0)),
-        width  = new_geo.width  and (new_geo.width  + (offset.width  or 0)),
-        height = new_geo.height and (new_geo.height + (offset.height or 0)),
+        x      = new_geo.x      and (new_geo.x + (offset.x or 0) + (m.left or 0) ),
+        y      = new_geo.y      and (new_geo.y + (offset.y or 0) + (m.top  or 0) ),
+        width  = new_geo.width  and math.max(
+            1, (new_geo.width  + (offset.width  or 0) - (m.left or 0) - (m.right or 0)  )
+        ),
+        height = new_geo.height and math.max(
+            1, (new_geo.height + (offset.height or 0) - (m.top  or 0) - (m.bottom or 0) )
+        ),
     }
 end
 
@@ -338,16 +361,13 @@ local function geometry_common(obj, args, new_geo, ignore_border_width)
 
             -- Apply the margins
             if args.margins then
-                local delta = type(args.margins) == "table" and args.margins or {
-                    left = args.margins , right  = args.margins,
-                    top  = args.margins , bottom = args.margins
-                }
+                local delta = get_decoration(args)
 
                 return {
-                    x      = dgeo.x      + (delta.left or 0),
-                    y      = dgeo.y      + (delta.top  or 0),
-                    width  = dgeo.width  - (delta.left or 0) - (delta.right  or 0),
-                    height = dgeo.height - (delta.top  or 0) - (delta.bottom or 0),
+                    x      = dgeo.x      - (delta.left or 0),
+                    y      = dgeo.y      - (delta.top  or 0),
+                    width  = dgeo.width  + (delta.left or 0) + (delta.right  or 0),
+                    height = dgeo.height + (delta.top  or 0) + (delta.bottom or 0),
                 }
             end
 
@@ -633,7 +653,7 @@ function placement.closest_corner(d, args)
     local new_args = setmetatable({position = corner}, {__index=args})
     local ngeo = placement_private.align(d, new_args)
 
-    return ngeo, corner
+    return fix_new_geometry(ngeo, args, true), corner
 end
 
 --- Place the client so no part of it will be outside the screen (workarea).
@@ -751,7 +771,7 @@ function placement.under_mouse(d, args)
     ngeo.width  = ngeo.width  - 2*bw
     ngeo.height = ngeo.height - 2*bw
 
-    return ngeo
+    return fix_new_geometry(ngeo, args, true)
 end
 
 --- Place the client next to the mouse.
@@ -862,7 +882,7 @@ function placement.resize_to_mouse(d, args)
 
     geometry_common(d, args, ngeo)
 
-    return ngeo
+    return fix_new_geometry(ngeo, args, true)
 end
 
 --- Move the drawable (client or wibox) `d` to a screen position or side.
@@ -913,7 +933,7 @@ function placement.align(d, args)
 
     attach(d, placement[args.position], args)
 
-    return ngeo
+    return fix_new_geometry(ngeo, args, true)
 end
 
 -- Add the alias functions
@@ -1002,7 +1022,7 @@ function placement.stretch(d, args)
 
     attach(d, placement["stretch_"..args.direction], args)
 
-    return ngeo
+    return fix_new_geometry(ngeo, args, true)
 end
 
 -- Add the alias functions
@@ -1056,7 +1076,7 @@ function placement.maximize(d, args)
 
     attach(d, placement.maximize, args)
 
-    return ngeo
+    return fix_new_geometry(ngeo, args, true)
 end
 
 -- Add the alias functions
@@ -1121,7 +1141,7 @@ function placement.scale(d, args)
 
     attach(d, placement.maximize, args)
 
-    return ngeo
+    return fix_new_geometry(ngeo, args, true)
 end
 
 ---@DOC_awful_placement_maximize_vertically_EXAMPLE@