Compute list index conversions independently of rendering

The model<->view index conversions were derived from arrays that only
renderLines populated. That made them depend on the list having been
rendered (so a conversion before the first render ignored the non-model
items), and it made them go stale whenever the model changed after a
render: converting an index then returned a wrong result, and once the
model had grown past the last rendered length the conversion indexed a
too-short array and panicked (seen in cherry_pick under -race).

The conversion is a pure function of the current list length and the
current non-model items, and needs none of the rendered display strings.
Compute it directly and drop the cached arrays, so the result is always
consistent with the current model and no longer depends on rendering.

searchModelCommits converts every commit's index, and building the
non-model items can be O(len) mid-rebase, so it would now be quadratic;
snapshot the non-model items once via modelToViewIndexConverter instead
of rebuilding them per index.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/gui/context/list_renderer.go

@@ -32,34 +32,76 @@ type ListRenderer struct {
 	getNonModelItems func() []*NonModelItem
 
 	// The remaining fields are private and shouldn't be initialized by clients
-	numNonModelItems        int
-	viewIndicesByModelIndex []int
-	modelIndicesByViewIndex []int
-	columnPositions         []int
+	columnPositions []int
 }
 
 func (self *ListRenderer) GetList() types.IList {
 	return self.list
 }
 
-func (self *ListRenderer) ModelIndexToViewIndex(modelIndex int) int {
-	modelIndex = lo.Clamp(modelIndex, 0, self.list.Len())
-	if self.viewIndicesByModelIndex != nil {
-		return self.viewIndicesByModelIndex[modelIndex]
+func (self *ListRenderer) getNonModelItemList() []*NonModelItem {
+	if self.getNonModelItems == nil {
+		return nil
 	}
+	return self.getNonModelItems()
+}
 
-	return modelIndex
+func (self *ListRenderer) ModelIndexToViewIndex(modelIndex int) int {
+	return modelIndexToViewIndex(self.list.Len(), self.getNonModelItemList(), modelIndex)
 }
 
 func (self *ListRenderer) ViewIndexToModelIndex(viewIndex int) int {
-	viewIndex = lo.Clamp(viewIndex, 0, self.list.Len()+self.numNonModelItems)
-	if self.modelIndicesByViewIndex != nil {
-		return self.modelIndicesByViewIndex[viewIndex]
-	}
+	return viewIndexToModelIndex(self.list.Len(), self.getNonModelItemList(), viewIndex)
+}
 
+// modelToViewIndexConverter returns a model-to-view index conversion that
+// reuses a single snapshot of the non-model items. Callers that convert many
+// indices in a row (e.g. search, which converts every commit) should use this
+// rather than calling ModelIndexToViewIndex per index, which would rebuild the
+// non-model items each time.
+func (self *ListRenderer) modelToViewIndexConverter() func(modelIndex int) int {
+	listLength := self.list.Len()
+	nonModelItems := self.getNonModelItemList()
+	return func(modelIndex int) int {
+		return modelIndexToViewIndex(listLength, nonModelItems, modelIndex)
+	}
+}
+
+// The view shows the model items with the non-model items (e.g. section
+// headers) inserted at their model indices. The two conversions below are
+// computed directly from the current list length and non-model items, so they
+// don't depend on the list having been rendered, and they can never be stale
+// with respect to a model that changed since the last render (which used to
+// cause both wrong results and index-out-of-range panics).
+//
+// The non-model items are assumed to be ordered by their Index, which is how
+// all producers build them; the i-th one therefore ends up at view index
+// Index+i.
+func modelIndexToViewIndex(listLength int, nonModelItems []*NonModelItem, modelIndex int) int {
+	modelIndex = lo.Clamp(modelIndex, 0, listLength)
+	// Each non-model item inserted at or before this model item pushes it down
+	// by one row in the view.
+	viewIndex := modelIndex
+	for _, item := range nonModelItems {
+		if item.Index <= modelIndex {
+			viewIndex++
+		}
+	}
 	return viewIndex
 }
 
+func viewIndexToModelIndex(listLength int, nonModelItems []*NonModelItem, viewIndex int) int {
+	viewIndex = lo.Clamp(viewIndex, 0, listLength+len(nonModelItems))
+	// Subtract the non-model items that appear before this view index.
+	modelIndex := viewIndex
+	for i, item := range nonModelItems {
+		if item.Index+i < viewIndex {
+			modelIndex--
+		}
+	}
+	return modelIndex
+}
+
 func (self *ListRenderer) ColumnPositions() []int {
 	return self.columnPositions
 }
@@ -71,23 +113,18 @@ func (self *ListRenderer) renderLines(startIdx int, endIdx int) string {
 	if self.getColumnAlignments != nil {
 		columnAlignments = self.getColumnAlignments()
 	}
-	nonModelItems := []*NonModelItem{}
-	self.numNonModelItems = 0
-	if self.getNonModelItems != nil {
-		nonModelItems = self.getNonModelItems()
-		self.prepareConversionArrays(nonModelItems)
-	}
+	nonModelItems := self.getNonModelItemList()
 	startModelIdx := 0
 	if startIdx == -1 {
 		startIdx = 0
 	} else {
-		startModelIdx = self.ViewIndexToModelIndex(startIdx)
+		startModelIdx = viewIndexToModelIndex(self.list.Len(), nonModelItems, startIdx)
 	}
 	endModelIdx := self.list.Len()
 	if endIdx == -1 {
 		endIdx = endModelIdx + len(nonModelItems)
 	} else {
-		endModelIdx = self.ViewIndexToModelIndex(endIdx)
+		endModelIdx = viewIndexToModelIndex(self.list.Len(), nonModelItems, endIdx)
 	}
 	lines, columnPositions := utils.RenderDisplayStrings(
 		self.getDisplayStrings(startModelIdx, endModelIdx),
@@ -97,23 +134,6 @@ func (self *ListRenderer) renderLines(startIdx int, endIdx int) string {
 	return strings.Join(lines, "\n")
 }
 
-func (self *ListRenderer) prepareConversionArrays(nonModelItems []*NonModelItem) {
-	self.numNonModelItems = len(nonModelItems)
-	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++ {
-			viewIndicesByModelIndex[i]++
-		}
-		modelIndicesByViewIndex = slices.Insert(
-			modelIndicesByViewIndex, item.Index+offset, modelIndicesByViewIndex[item.Index+offset])
-		offset++
-	}
-	self.viewIndicesByModelIndex = viewIndicesByModelIndex
-	self.modelIndicesByViewIndex = modelIndicesByViewIndex
-}
-
 func (self *ListRenderer) insertNonModelItems(
 	nonModelItems []*NonModelItem, endIdx int, startIdx int, lines []string, columnPositions []int,
 ) []string {

pkg/gui/context/list_renderer_test.go

@@ -254,9 +254,6 @@ func TestListRenderer_ModelIndexToViewIndex_and_back(t *testing.T) {
 				getNonModelItems: getNonModelItems,
 			}
 
-			// Need to render first so that it knows the non-model items
-			self.renderLines(-1, -1)
-
 			for i := range len(s.modelIndices) {
 				assert.Equal(t, s.expectedViewIndices[i], self.ModelIndexToViewIndex(s.modelIndices[i]))
 			}
@@ -288,10 +285,6 @@ func TestListRenderer_IndexConversionsAreRenderIndependent(t *testing.T) {
 	}
 
 	// Deliberately convert without rendering first.
-	/* EXPECTED:
 	assert.Equal(t, 2, self.ModelIndexToViewIndex(1))
 	assert.Equal(t, 1, self.ViewIndexToModelIndex(2))
-	ACTUAL: */
-	assert.Equal(t, 1, self.ModelIndexToViewIndex(1))
-	assert.Equal(t, 2, self.ViewIndexToModelIndex(2))
 }

pkg/gui/context/local_commits_context.go

@@ -224,7 +224,7 @@ func (self *LocalCommitsContext) RefForAdjustingLineNumberInDiff() string {
 }
 
 func (self *LocalCommitsContext) ModelSearchResults(searchStr string, caseSensitive bool) []gocui.SearchPosition {
-	return searchModelCommits(caseSensitive, self.GetCommits(), self.ColumnPositions(), self.ModelIndexToViewIndex, searchStr)
+	return searchModelCommits(caseSensitive, self.GetCommits(), self.ColumnPositions(), self.modelToViewIndexConverter(), searchStr)
 }
 
 func (self *LocalCommitsViewModel) SetLimitCommits(value bool) {

pkg/gui/context/sub_commits_context.go

@@ -223,7 +223,7 @@ func (self *SubCommitsContext) RefForAdjustingLineNumberInDiff() string {
 }
 
 func (self *SubCommitsContext) ModelSearchResults(searchStr string, caseSensitive bool) []gocui.SearchPosition {
-	return searchModelCommits(caseSensitive, self.GetCommits(), self.ColumnPositions(), self.ModelIndexToViewIndex, searchStr)
+	return searchModelCommits(caseSensitive, self.GetCommits(), self.ColumnPositions(), self.modelToViewIndexConverter(), searchStr)
 }
 
 func (self *SubCommitsContext) IndexForGotoBottom() int {