Do not leave the current area when re-checking the coverage area.

common/area.go

@@ -62,7 +62,7 @@ func UncoveredAreas(r *Renderer, newAreas, oldAreas []Area) []Area {
 							// Extension in x possible?
 							if extendDirs&1 == 1 {
 								nx := area.X2 + 1
-								if nx >= int16(r.width) {
+								if nx > oldArea.X2 {
 									extendDirs &= ^1
 									continue
 								}
@@ -79,7 +79,7 @@ func UncoveredAreas(r *Renderer, newAreas, oldAreas []Area) []Area {
 						} else if extendDirs&2 == 2 {
 							// Symmetric case in z direction
 							nz := area.Z2 + 1
-							if nz >= int16(r.height) {
+							if nz > oldArea.Z2 {
 								extendDirs &= ^2
 								continue
 							}