diff --git a/pkg/commands/patch/patch_builder.go b/pkg/commands/patch/patch_builder.go
index 0d5ca34f8..890a53518 100644
--- a/pkg/commands/patch/patch_builder.go
+++ b/pkg/commands/patch/patch_builder.go
@@ -299,6 +299,76 @@ func (p *PatchBuilder) GetFileIncLineIndices(filename string, previousPath strin
 	return info.includedLineIndices, nil
 }
 
+// LineIdentity identifies a change line (an addition or deletion) by its file line
+// number and whether it's a deletion, independently of the line's index in the parsed
+// patch. It is the identity the diff-line metadata resolves a rendered row to (see
+// types.DiffLineInfo.PatchSelectLine), and lets the focused main view toggle patch
+// membership and drive the inclusion gutter without dealing in patch-line indices —
+// which differ between the raw diff and however a pager renders it.
+type LineIdentity struct {
+	LineNumber int
+	IsDeletion bool
+}
+
+// changeLineIndexByIdentity scans a parsed diff and returns, for each change line, its
+// index in the patch keyed by the line's identity. An addition is keyed by its new-file
+// line number, a deletion by its old-file line number, so each change line has a
+// distinct identity (two consecutive deletions share a new-file number but differ in
+// the old-file one).
+func changeLineIndexByIdentity(parsed *Patch) map[LineIdentity]int {
+	byIdentity := map[LineIdentity]int{}
+	for idx, line := range parsed.Lines() {
+		switch {
+		case line.IsAddition():
+			byIdentity[LineIdentity{parsed.LineNumberOfLine(idx), false}] = idx
+		case line.IsDeletion():
+			byIdentity[LineIdentity{parsed.OldLineNumberOfLine(idx), true}] = idx
+		}
+	}
+	return byIdentity
+}
+
+// PatchLineIndicesForLines maps the given change-line identities to their indices in
+// filename's parsed diff — the index form that AddFileLineRange / RemoveFileLineRange
+// and GetFileIncLineIndices work in. Identities that don't correspond to a change line
+// (e.g. a context line) are skipped. It is how the focused main view, which knows a
+// selection only as metadata identities, drives patch building.
+func (p *PatchBuilder) PatchLineIndicesForLines(filename string, lines []LineIdentity) ([]int, error) {
+	info, err := p.getFileInfo(filename, "")
+	if err != nil {
+		return nil, err
+	}
+	byIdentity := changeLineIndexByIdentity(Parse(info.diff))
+	indices := make([]int, 0, len(lines))
+	for _, line := range lines {
+		if idx, ok := byIdentity[line]; ok {
+			indices = append(indices, idx)
+		}
+	}
+	return indices, nil
+}
+
+// IncludedLineIdentities returns the identities of the change lines currently included
+// in the patch for filename — the identity space the inclusion gutter matches rendered
+// rows against. Empty when the file isn't part of the patch.
+func (p *PatchBuilder) IncludedLineIdentities(filename string) []LineIdentity {
+	info, ok := p.fileInfoMap[filename]
+	if !ok || info.mode == UNSELECTED {
+		return nil
+	}
+	includedIdx := make(map[int]bool, len(info.includedLineIndices))
+	for _, idx := range info.includedLineIndices {
+		includedIdx[idx] = true
+	}
+	var identities []LineIdentity
+	for identity, idx := range changeLineIndexByIdentity(Parse(info.diff)) {
+		if includedIdx[idx] {
+			identities = append(identities, identity)
+		}
+	}
+	return identities
+}
+
 // clears the patch
 func (p *PatchBuilder) Reset() {
 	p.mutex.Lock()
diff --git a/pkg/commands/patch/patch_builder_test.go b/pkg/commands/patch/patch_builder_test.go
new file mode 100644
index 000000000..46d173eb1
--- /dev/null
+++ b/pkg/commands/patch/patch_builder_test.go
@@ -0,0 +1,52 @@
+package patch
+
+import (
+	"testing"
+
+	"github.com/sirupsen/logrus"
+	"github.com/stretchr/testify/assert"
+)
+
+// newTestPatchBuilder returns a PatchBuilder whose files all resolve to the given
+// diff, started for a dummy commit.
+func newTestPatchBuilder(diff string) *PatchBuilder {
+	pb := NewPatchBuilder(logrus.New().WithField("test", "test"),
+		func(from, to string, reverse bool, filename string, previousPath string, plain bool) (string, error) {
+			return diff, nil
+		})
+	pb.Start("from", "to", false, true)
+	return pb
+}
+
+// In simpleDiff the deletion "-orange" is patch line index 6 (old-file line 2) and the
+// addition "+grape" is index 7 (new-file line 2).
+func TestPatchLineIndicesForLines(t *testing.T) {
+	pb := newTestPatchBuilder(simpleDiff)
+
+	indices, err := pb.PatchLineIndicesForLines("filename", []LineIdentity{
+		{LineNumber: 2, IsDeletion: true},  // -orange
+		{LineNumber: 2, IsDeletion: false}, // +grape
+		{LineNumber: 1, IsDeletion: false}, // " apple" — a context line, no change index
+	})
+	assert.NoError(t, err)
+	assert.Equal(t, []int{6, 7}, indices, "context-line identity is skipped; change lines map to their indices")
+}
+
+func TestIncludedLineIdentities(t *testing.T) {
+	pb := newTestPatchBuilder(simpleDiff)
+
+	// Nothing included yet.
+	assert.Empty(t, pb.IncludedLineIdentities("filename"))
+
+	// Include only the deletion: it comes back as its identity, the addition does not.
+	assert.NoError(t, pb.AddFileLineRange("filename", "", []int{6}))
+	assert.Equal(t,
+		[]LineIdentity{{LineNumber: 2, IsDeletion: true}},
+		pb.IncludedLineIdentities("filename"))
+
+	// Including the addition too yields both identities (order-independent).
+	assert.NoError(t, pb.AddFileLineRange("filename", "", []int{7}))
+	assert.ElementsMatch(t,
+		[]LineIdentity{{LineNumber: 2, IsDeletion: true}, {LineNumber: 2, IsDeletion: false}},
+		pb.IncludedLineIdentities("filename"))
+}