Make prepareConversionArrays a little more concurrency safe

I have seen cases where during a rebase (two nonModelItems) all entries in
viewIndicesByModelIndex beyond the second nonModelItem were off by 4 rather than
2 as I would expect. The only explanation I have for this is that the function
was called concurrently.

Improve this by working on a local variable and only assign to self at the end;
this is not a real fix for the concurrency issue of course, but it makes it much
less likely to be a problem in practice.

Author: stefanhaller

pkg/gui/context/list_renderer.go

@@ -99,17 +99,19 @@ func (self *ListRenderer) renderLines(startIdx int, endIdx int) string {
 
 func (self *ListRenderer) prepareConversionArrays(nonModelItems []*NonModelItem) {
 	self.numNonModelItems = len(nonModelItems)
-	self.viewIndicesByModelIndex = lo.Range(self.list.Len() + 1)
-	self.modelIndicesByViewIndex = lo.Range(self.list.Len() + 1)
+	viewIndicesByModelIndex := lo.Range(self.list.Len() + 1)
+	modelIndicesByViewIndex := lo.Range(self.list.Len() + 1)
 	offset := 0
 	for _, item := range nonModelItems {
 		for i := item.Index; i <= self.list.Len(); i++ {
-			self.viewIndicesByModelIndex[i]++
+			viewIndicesByModelIndex[i]++
 		}
-		self.modelIndicesByViewIndex = slices.Insert(
-			self.modelIndicesByViewIndex, item.Index+offset, self.modelIndicesByViewIndex[item.Index+offset])
+		modelIndicesByViewIndex = slices.Insert(
+			modelIndicesByViewIndex, item.Index+offset, modelIndicesByViewIndex[item.Index+offset])
 		offset++
 	}
+	self.viewIndicesByModelIndex = viewIndicesByModelIndex
+	self.modelIndicesByViewIndex = modelIndicesByViewIndex
 }
 
 func (self *ListRenderer) insertNonModelItems(