gears.wallpaper: Handle concurrent screen changes

The code in gears.wallpaper currently sets a wallpaper in a deferred fashion.
Only a while after it is told to do something does it actually do the wallpaper
change. This is to incorporate many wallpaper changes right after another. These
changes happens during startup where the wallpaper for each screen is set one
after another.

However, since we no longer restart on RandR changes, the screen configuration
could change while we have a pending wallpaper. In this case, part of the
wallpaper could be "chopped off", because the surface that we draw the wallpaper
to is too small.

This commit makes gears.wallpaper track the size of the pending wallpaper and
create a new surface if the already-pending one is too small.

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

lib/gears/wallpaper.lua

@@ -18,7 +18,7 @@ local function root_geometry()
     return { x = 0, y = 0, width = width, height = height }
 end
 
--- A cairo surface that we still want to set as the wallpaper
+-- Information about a pending wallpaper change, see prepare_context()
 local pending_wallpaper = nil
 
 --- Prepare the needed state for setting a wallpaper.
@@ -29,35 +29,49 @@ local pending_wallpaper = nil
 -- @return[1] The available geometry (table with entries width and height)
 -- @return[1] A cairo context that the wallpaper should be drawn to
 function wallpaper.prepare_context(s)
+    local root_width, root_height = root.size()
     local geom = s and screen[s].geometry or root_geometry()
-    local cr
+    local source, target, cr
 
     if not pending_wallpaper then
         -- Prepare a pending wallpaper
-        local wp = surface(root.wallpaper())
+        source = surface(root.wallpaper())
-
+        target = source:create_similar(cairo.Content.COLOR, root_width, root_height)
-        pending_wallpaper = wp:create_similar(cairo.Content.COLOR, root.size())
-
-        -- Copy the old wallpaper to the new one
-        cr = cairo.Context(pending_wallpaper)
-        cr:save()
-        cr.operator = cairo.Operator.SOURCE
-        cr:set_source_surface(wp, 0, 0)
-        cr:paint()
-        cr:restore()
 
         -- Set the wallpaper (delayed)
         timer.delayed_call(function()
             local paper = pending_wallpaper
             pending_wallpaper = nil
-            wallpaper.set(paper)
+            wallpaper.set(paper.surface)
-            paper:finish()
+            paper.surface:finish()
         end)
+    elseif root_width > pending_wallpaper.width or root_height > pending_wallpaper.height then
+        -- The root window was resized while a wallpaper is pending
+        source = pending_wallpaper.surface
+        target = source:create_similar(cairo.Content.COLOR, root_width, root_height)
     else
         -- Draw to the already-pending wallpaper
-        cr = cairo.Context(pending_wallpaper)
+        source = nil
+        target = pending_wallpaper.surface
     end
 
+    cr = cairo.Context(target)
+
+    if source then
+        -- Copy the old wallpaper to the new one
+        cr:save()
+        cr.operator = cairo.Operator.SOURCE
+        cr:set_source_surface(source, 0, 0)
+        cr:paint()
+        cr:restore()
+    end
+
+    pending_wallpaper = {
+        surface = target,
+        width = root_width,
+        height = root_height
+    }
+
     -- Only draw to the selected area
     cr:translate(geom.x, geom.y)
     cr:rectangle(0, 0, geom.width, geom.height)