layer-shell: start transaction on map

The fact that this call is missing is a bug, as the changes made by
arranging the output layers as well as changes to the focus will not be
fully applied.

 river/LayerSurface.zig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/river/LayerSurface.zig b/river/LayerSurface.zig
index 239543c..4231bad 100644
--- a/river/LayerSurface.zig
+++ b/river/LayerSurface.zig
@@ -102,6 +102,7 @@ fn handleMap(listener: *wl.Listener(*wlr.LayerSurfaceV1), wlr_layer_surface: *wl
     const node = @fieldParentPtr(std.TailQueue(Self).Node, "data", self);
     self.output.getLayer(self.layer).append(node);
     self.output.arrangeLayers(.mapped);
+    server.root.startTransaction();
 }
 
 fn handleUnmap(listener: *wl.Listener(*wlr.LayerSurfaceV1), _: *wlr.LayerSurfaceV1) void {