Clamp the commit-file tree selection when the tree is rebuilt

CommitFileTreeViewModel embedded the low-level tree's SetTree, which
rebuilds the node list without touching the cursor. So after a shrinking
rebuild (e.g. moving a patch out into the index removes a file), the
selection index could be left past the end of the tree. GetSelectedItems
then indexes out of range and returns a nil node, which segfaults callers
such as canEditFiles when the options map is rendered during layout.

Override SetTree to ClampSelection after the rebuild. Unlike
FileTreeViewModel we deliberately don't also re-find the selected node by
path: that walk lands on the containing directory when a file is removed
from a dir that then collapses, whereas keeping the clamped index lands
on the sibling file (see discard_old_file_changes).

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

pkg/gui/filetree/commit_file_tree_view_model.go

@@ -142,6 +142,22 @@ func (self *CommitFileTreeViewModel) GetSelectedPath() string {
 	return node.GetPath()
 }
 
+// SetTree rebuilds the tree and clamps the selection so it stays in range. The
+// embedded tree's SetTree only rebuilds the node list and doesn't touch the
+// cursor, so after a shrinking rebuild (e.g. moving a patch out into the index)
+// the selection index could be left past the end of the tree; GetSelectedItems
+// would then return a nil node and crash callers such as canEditFiles when the
+// options map is rendered during layout.
+//
+// Unlike FileTreeViewModel.SetTree we don't re-find the selected node by path
+// afterwards: that walk lands on the containing directory when a file is removed
+// from a dir that then collapses, whereas keeping the (clamped) index lands on
+// the sibling file, which is what we want here.
+func (self *CommitFileTreeViewModel) SetTree() {
+	self.ICommitFileTree.SetTree()
+	self.ClampSelection()
+}
+
 // duplicated from file_tree_view_model.go. Generics will help here
 func (self *CommitFileTreeViewModel) ToggleShowTree() {
 	selectedNode := self.GetSelected()

pkg/gui/filetree/commit_file_tree_view_model_test.go

@@ -0,0 +1,40 @@
+package filetree
+
+import (
+	"testing"
+
+	"github.com/jesseduffield/lazygit/pkg/commands/models"
+	"github.com/jesseduffield/lazygit/pkg/common"
+	"github.com/stretchr/testify/assert"
+)
+
+// When the tree shrinks under the selection - e.g. moving a patch out into the
+// index removes a file - SetTree must keep the selection in range. Otherwise
+// GetSelectedItems returns a nil node, which crashes callers such as
+// canEditFiles when the options map is rendered during layout.
+func TestCommitFileTreeViewModelSetTreeClampsSelectionOnShrink(t *testing.T) {
+	files := []*models.CommitFile{
+		{Path: "file1"},
+		{Path: "file2"},
+		{Path: "file3"},
+	}
+	viewModel := NewCommitFileTreeViewModel(
+		func() []*models.CommitFile { return files },
+		common.NewDummyCommon(),
+		false, // flat list
+	)
+	viewModel.SetTree()
+	viewModel.SetSelectedLineIdx(viewModel.Len() - 1)
+
+	// The file under the cursor goes away and the tree shrinks.
+	files = []*models.CommitFile{{Path: "file1"}}
+	viewModel.SetTree()
+
+	assert.Less(t, viewModel.GetSelectedLineIdx(), viewModel.Len())
+	assert.NotNil(t, viewModel.GetSelected())
+	items, _, _ := viewModel.GetSelectedItems()
+	assert.NotEmpty(t, items)
+	for _, item := range items {
+		assert.NotNil(t, item)
+	}
+}