awful.layout: Split the layout parameters computation from 'arrange'

It is necessary to have it beforehand when creating layout objects
for unselected layouts.

In the current layout system, there is no layout object, but to allow
tabs and dynamic tagging features like ion3, layouts cannot be stateless.

lib/awful/layout/init.lua

@@ -14,6 +14,7 @@ local util = require("awful.util")
 local ascreen = require("awful.screen")
 local capi = {
     screen = screen,
+    mouse  = mouse,
     awesome = awesome,
     client = client,
     tag = tag
@@ -79,7 +80,7 @@ function layout.inc(i, s, layouts)
         local curlayout = layout.get(s)
         local curindex
         for k, v in ipairs(layouts) do
-            if v == curlayout then
+            if v == curlayout or curlayout._type == v then
                 curindex = k
                 break
             end
@@ -109,6 +110,54 @@ function layout.set(_layout, t)
     tag.setproperty(t, "layout", _layout)
 end
 
+--- Get the layout parameters used for the screen
+--
+-- This should give the same result as "arrange", but without the "geometries"
+-- parameter, as this is computed during arranging.
+--
+-- If `t` is given, `screen` is ignored, if none are given, the mouse screen is
+-- used.
+--
+-- @tparam[opt] tag t The tag to query
+-- @param[opt] screen The screen
+-- @treturn table A table with the workarea (x, y, width, height), the screen
+--   geometry (x, y, width, height), the clients, the screen and sometime, a
+--   "geometries" table with client as keys and geometry as value
+function layout.parameters(t, screen)
+    t = t or tag.selected(screen)
+
+    if not t then return end
+
+    screen = tag.getscreen(t) or 1
+
+    local p = {}
+
+    p.workarea = capi.screen[screen].workarea
+
+    local useless_gap = tag.getgap(t, #client.tiled(screen))
+
+    -- Handle padding
+    local padding = ascreen.padding(capi.screen[screen]) or {}
+
+    p.workarea.x = p.workarea.x + (padding.left or 0) + useless_gap
+
+    p.workarea.y = p.workarea.y + (padding.top or 0) + useless_gap
+
+    p.workarea.width = p.workarea.width - ((padding.left or 0 ) +
+        (padding.right or 0) + useless_gap * 2)
+
+    p.workarea.height = p.workarea.height - ((padding.top or 0) +
+        (padding.bottom or 0) + useless_gap * 2)
+
+    p.geometry    = capi.screen[screen].geometry
+    p.clients     = client.tiled(screen)
+    p.screen      = screen
+    p.padding     = padding
+    p.useless_gap = useless_gap
+
+    return p
+end
+
 --- Arrange a screen using its current layout.
 -- @param screen The screen to arrange.
 function layout.arrange(screen)
@@ -119,22 +168,10 @@ function layout.arrange(screen)
         if arrange_lock then return end
         arrange_lock = true
 
-        local p = {}
+        local p = layout.parameters(nil, screen)
-        p.workarea = capi.screen[screen].workarea
+
-        local useless_gap = tag.getgap(tag.selected(screen),
+        local useless_gap = p.useless_gap
-                                       #client.tiled(screen))
-        -- Handle padding
-        local padding = ascreen.padding(capi.screen[screen]) or {}
-        p.workarea.x = p.workarea.x + (padding.left or 0) + useless_gap
-        p.workarea.y = p.workarea.y + (padding.top or 0) + useless_gap
-        p.workarea.width = p.workarea.width - ((padding.left or 0 ) +
-            (padding.right or 0) + useless_gap * 2)
-        p.workarea.height = p.workarea.height - ((padding.top or 0) +
-            (padding.bottom or 0) + useless_gap * 2)
 
-        p.geometry = capi.screen[screen].geometry
-        p.clients = client.tiled(screen)
-        p.screen = screen
         p.geometries = setmetatable({}, {__mode = "k"})
         layout.get(screen).arrange(p)
         for c, g in pairs(p.geometries) do