naughty: skip permanent notifications when there is no more room

This skips permanent notifications (with timeout=0) in `get_offset` when
there is not enough room for a new notification.  It will still fallback
to removing the first/oldest one.

Closes https://github.com/awesomeWM/awesome/pull/306.

lib/naughty/core.lua

@@ -209,10 +209,24 @@ local function get_offset(screen, position, idx, width, height)
         v.y = ws.y + ws.height - (naughty.config.padding + height + existing)
     end
 
+    -- Find old notification to replace in case there is not enough room.
+    -- This tries to skip permanent notifications (without a timeout),
+    -- e.g. critical ones.
+    local find_old_to_replace = function()
+        for i = 1, idx-1 do
+            local n = naughty.notifications[screen][position][i]
+            if n.timeout > 0 then
+                return n
+            end
+        end
+        -- Fallback to first one.
+        return naughty.notifications[screen][position][1]
+    end
+
     -- if positioned outside workarea, destroy oldest popup and recalculate
     if v.y + height > ws.y + ws.height or v.y < ws.y then
+        naughty.destroy(find_old_to_replace())
         idx = idx - 1
-        naughty.destroy(naughty.notifications[screen][position][1])
         v = get_offset(screen, position, idx, width, height)
     end
     if not v.idx then v.idx = idx end