Game Development Reference
Make sure your Layer Mask is selected. Go to the Bucket Fill tool.
Select the Fill Whole Selecion buton as shown in the following screenshot, and
click in the selected area.
What just happened?
You expanded your selecion area by a single pixel and then modiied the layer mask to
include the new selecion area. This meant that any white lines at the edges of the cobbled
areas were removed. You can repeat this now for any other affected areas.