Makes awful.screen.focus keep the cursor position on screen

When changing focus to an other screen, awful.screen.focus keeps relative
position of the cursor, instead of moving to the top left corner. Does not
trigger mouse:enter and mouse:leave signals.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/screen.lua.in

@@ -37,14 +37,28 @@ function screen.getbycoord(x, y)
     end
 end
 
---- Give the focus to a screen, and move pointer.
+--- Give the focus to a screen, and move pointer. Keeps relative position of the pointer on the screen.
 -- @param screen Screen number.
 function screen.focus(_screen)
     if _screen > capi.screen.count() then _screen = capi.mouse.screen end
+
+    -- screen and pos for current screen
+    local s = capi.mouse.screen
+    local pos = capi.mouse.coords()
+
+    -- keep relative mouse position on the new screen
+    local relx = (pos.x - capi.screen[s].geometry.x) / capi.screen[s].geometry.width
+    local rely = (pos.y - capi.screen[s].geometry.y) / capi.screen[s].geometry.height
+
+    pos.x = capi.screen[_screen].geometry.x + relx * capi.screen[_screen].geometry.width
+    pos.y = capi.screen[_screen].geometry.y + rely * capi.screen[_screen].geometry.height
+
+   -- move cursor without triggering signals mouse::enter and mouse::leave
+    capi.mouse.screen = _screen
+    capi.mouse.coords(pos, true)
+
     local c = client.focus.history.get(_screen, 0)
     if c then capi.client.focus = c end
-    -- Move the mouse on the screen
-    capi.mouse.screen = _screen
 end
 
 --- Give the focus to a screen, and move pointer, by physical position relative to current screen.