Keep your place in the diff when changing the context size

Pressing { or } re-rendered the diff with less or more context around each
change and dropped you back at the top of it, so finding your way back to
what you were reading was on you.

Remember where the view is before triggering the re-render, and have the
restore put it back there. The line to remember is the selected one when a
selection is showing, and the middle visible line otherwise — what you are
looking at, rather than the view's top edge. It is remembered by identity,
because the new rendering puts it on a different line of the view, and it
is found again by the records a diff renderer states for its rows, or by
parsing the rendering as a diff where it still looks like one.

The line may not be there at all afterwards: shrinking the context takes
context lines away. So the lines around it come along as fallbacks, nearest
first, and the view lands on the nearest one that survived, put back on the
screen row it was on — leaving what you were reading where it was, give or
take the line that went. The walk that collects them covers the whole diff
rather than stopping at the nearest change on either side: those always
survive a context-size change, but not everything a re-render can do to a
diff is that considerate.

Under a diff renderer that says nothing about its rows there is nothing to
look for at all, and the view is left at the offset it had instead: often not
the line it was on, but always closer to it than the top of the diff. That is
what the re-render asks for besides the restore, since it is a different
command from the one that produced what is on screen and would otherwise be
taken for a diff the user has never seen.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

pkg/gocui/view.go

@@ -2339,6 +2339,11 @@ func (v *View) SelectedLineIdx() int {
 	return seletedLineIdx
 }
 
+// IsLineVisible reports whether the given view line is one of those on screen.
+func (v *View) IsLineVisible(viewLine int) bool {
+	return viewLine >= v.OriginY() && viewLine < v.OriginY()+v.InnerHeight()
+}
+
 // MiddleVisibleLineIdx returns the view line halfway down the visible content. It
 // stands in for a cursor in a view that has none: of the lines on screen, the one in
 // the middle is the likeliest to be the one being read.

pkg/gui/controllers/context_lines_controller.go

@@ -83,6 +83,11 @@ func (self *ContextLinesController) applyChange() error {
 	case context.STAGING_MAIN_CONTEXT_KEY, context.STAGING_SECONDARY_CONTEXT_KEY:
 		self.c.Refresh(types.RefreshOptions{Scope: []types.RefreshableView{types.STAGING}})
 	default:
+		// The diff is about to be rendered again with more or less context around
+		// each change, which reads as the lines you were looking at moving up or down
+		// the view; keep them where they are instead.
+		self.c.Helpers().DiffLine.PreserveDiffPositionOnRerender(self.c.Contexts().Normal.GetView())
+		self.c.Helpers().DiffLine.PreserveDiffPositionOnRerender(self.c.Contexts().NormalSecondary.GetView())
 		currentContext.HandleRenderToMain()
 	}
 	return nil

pkg/gui/controllers/helpers/diff_line_helper.go

@@ -66,14 +66,21 @@ func (self *DiffLineHelper) GetDiffLineInfo(view *gocui.View, viewLineIdx int) (
 // (a side-by-side row shows a deletion and the addition replacing it); the leftmost
 // is the one a reader would call the row's own, so it is the row's identity.
 func (self *DiffLineHelper) diffLineInfoFromRecords(metadata []string) (types.DiffLineInfo, bool) {
-	if len(metadata) == 0 {
+	identities := self.diffLineIdentitiesFromRecords(metadata)
+	if len(identities) == 0 {
 		return types.DiffLineInfo{}, false
 	}
-	parsed, ok := parseDiffLineMetadata(metadata[0])
-	if !ok {
-		return types.DiffLineInfo{}, false
-	}
-	return self.diffLineInfo(parsed), true
+	return identities[0], true
+}
+
+// diffLineIdentitiesFromRecords recovers the identity of every diff line the row's
+// records state, left to right. Which of them a reader is after depends on the
+// reader: the one the row leads with is the row's own identity (see
+// diffLineInfoFromRecords), while a reader looking for a particular line has to
+// consider them all, since which of a modification's two halves leads a row is up to
+// the rendering.
+func (self *DiffLineHelper) diffLineIdentitiesFromRecords(metadata []string) []types.DiffLineInfo {
+	return self.diffLineInfos(parseDiffLineRecords(metadata))
 }
 
 // resolvedDiffLine is one rendered row's recovered identity, plus whether it could
@@ -86,36 +93,59 @@ type resolvedDiffLine struct {
 // resolveDiffLines recovers the identity of every row of a rendered diff in one
 // pass, indexed 1:1 with contents. It is the batch form of GetDiffLineInfo, for the
 // whole-buffer scans (which change lines are where, which file each row belongs
-// to), and reads the rendering the same way: by the renderer's records, or by
-// parsing it as a unified diff (see renderingStatesDiffLines). Resolving row by row
-// would re-run the buffer parser's whole-section parse once per row — O(n²) on a
-// large single-file diff — so the buffer parser runs once for the whole buffer.
+// to). A row's identity is the line it leads with, of those resolveDiffLineIdentities
+// finds on it.
 func (self *DiffLineHelper) resolveDiffLines(contents []gocui.DiffLineContent) []resolvedDiffLine {
 	resolved := make([]resolvedDiffLine, len(contents))
-	if renderingStatesDiffLines(contents) {
-		for i, content := range contents {
-			if info, ok := self.diffLineInfoFromRecords(content.Metadata); ok {
-				resolved[i] = resolvedDiffLine{info, true}
-			}
-		}
-		return resolved
-	}
-
-	for i, parsed := range parseAllDiffLinesFromBuffer(diffLineTexts(contents)) {
-		if parsed.ok {
-			resolved[i] = resolvedDiffLine{self.diffLineInfo(parsed.parsed), true}
+	for i, identities := range self.resolveDiffLineIdentities(contents) {
+		if len(identities) > 0 {
+			resolved[i] = resolvedDiffLine{identities[0], true}
 		}
 	}
 	return resolved
 }
 
+// resolveDiffLineIdentities recovers every diff line each row of a rendered diff
+// shows, in one pass, indexed 1:1 with contents. It reads the rendering the way
+// GetDiffLineInfo does, by the renderer's records or by parsing it as a unified diff
+// (see parseDiffLineIdentities), and is the form of the batch resolver for the
+// readers that can't settle for the line a row leads with: looking for a remembered
+// line in a new rendering has to consider both halves of a modification, since a
+// side-by-side row leads with the deletion whose addition was what got remembered
+// under a unified one.
+func (self *DiffLineHelper) resolveDiffLineIdentities(contents []gocui.DiffLineContent) [][]types.DiffLineInfo {
+	identities := make([][]types.DiffLineInfo, len(contents))
+	for i, parsed := range parseDiffLineIdentities(contents) {
+		if len(parsed) > 0 {
+			identities[i] = self.diffLineInfos(parsed)
+		}
+	}
+	return identities
+}
+
 // diffLineInfo turns a parser's result into the absolute-path identity consumers
 // work with. The path arrives repo-relative from the diff header, but a renderer
 // states it however it likes, absolute paths included.
 func (self *DiffLineHelper) diffLineInfo(parsed parsedDiffLine) types.DiffLineInfo {
+	return diffLineInfoIn(self.c.Git().RepoPaths.WorktreePath(), parsed)
+}
+
+// diffLineInfos is diffLineInfo over every line of a row.
+func (self *DiffLineHelper) diffLineInfos(parsed []parsedDiffLine) []types.DiffLineInfo {
+	infos := make([]types.DiffLineInfo, len(parsed))
+	for i, line := range parsed {
+		infos[i] = self.diffLineInfo(line)
+	}
+	return infos
+}
+
+// diffLineInfoIn is diffLineInfo against a given worktree, for the callers that can't
+// ask which repo we are in where they run: a repo switch replaces it, so only the UI
+// thread may read it.
+func diffLineInfoIn(worktreePath string, parsed parsedDiffLine) types.DiffLineInfo {
 	path := parsed.Path
 	if !filepath.IsAbs(path) {
-		path = filepath.Join(self.c.Git().RepoPaths.WorktreePath(), path)
+		path = filepath.Join(worktreePath, path)
 	}
 
 	return types.DiffLineInfo{

pkg/gui/controllers/helpers/diff_line_parser.go

@@ -125,6 +125,46 @@ func renderingStatesDiffLines(contents []gocui.DiffLineContent) bool {
 	})
 }
 
+// parseDiffLineRecords parses the records a row carries, left to right, leaving out
+// the ones we don't understand. A row carries more than one when the rendering puts
+// two diff lines on it, as a side-by-side row does with a deletion and the addition
+// replacing it.
+func parseDiffLineRecords(metadata []string) []parsedDiffLine {
+	parsed := make([]parsedDiffLine, 0, len(metadata))
+	for _, record := range metadata {
+		if line, ok := parseDiffLineMetadata(record); ok {
+			parsed = append(parsed, line)
+		}
+	}
+	return parsed
+}
+
+// parseDiffLineIdentities recovers, for every row of a rendering, the diff lines it
+// shows, indexed 1:1 with contents; a row that shows none we can place gets an empty
+// entry. The rendering is read the way renderingStatesDiffLines settles: by the
+// renderer's records, every one a row carries, or else by parsing the rendering as a
+// unified diff, where each row shows one line. Each file's section is parsed once;
+// resolving row by row would re-run that parse once per row, O(n²) on a large
+// single-file diff.
+func parseDiffLineIdentities(contents []gocui.DiffLineContent) [][]parsedDiffLine {
+	identities := make([][]parsedDiffLine, len(contents))
+	if renderingStatesDiffLines(contents) {
+		for i, content := range contents {
+			if parsed := parseDiffLineRecords(content.Metadata); len(parsed) > 0 {
+				identities[i] = parsed
+			}
+		}
+		return identities
+	}
+
+	for i, parsed := range parseAllDiffLinesFromBuffer(diffLineTexts(contents)) {
+		if parsed.ok {
+			identities[i] = []parsedDiffLine{parsed.parsed}
+		}
+	}
+	return identities
+}
+
 // fileSectionBounds returns the half-open range [start, end) of the file section
 // containing targetIdx: the nearest line starting a section at or above it, up to
 // where that section ends. start is -1 when targetIdx is above the first file

pkg/gui/controllers/helpers/diff_line_parser_test.go

@@ -420,3 +420,61 @@ func TestRenderingStatesDiffLines(t *testing.T) {
 		})
 	}
 }
+
+func TestParseDiffLineIdentities(t *testing.T) {
+	row := func(text string, records ...string) gocui.DiffLineContent {
+		return gocui.DiffLineContent{Text: text, Metadata: records}
+	}
+
+	t.Run("a rendering with records is read by them alone", func(t *testing.T) {
+		// A renderer's picture of a commit that adds a test whose input is a diff. The
+		// test's "diff --git" line is an added line of the test's file and is shown on
+		// a row of its own, and the rows the renderer puts between hunks carry no
+		// record. Parsed as a diff, that row would open a section of a file the diff
+		// doesn't have and claim every untagged row below it.
+		contents := []gocui.DiffLineContent{
+			row("src/parser.rs", "1;f;;;src/parser.rs"),
+			row(`let input = "\`, "1;c;10;;src/parser.rs"),
+			row("diff --git a/img.png b/img.png", "1;a;11;;src/parser.rs"),
+			row("Binary files a/img.png and b/img.png differ", "1;a;12;;src/parser.rs"),
+			row(""),
+			row("fn later() {}", "1;c;40;;src/parser.rs"),
+		}
+
+		assert.Equal(t, [][]parsedDiffLine{
+			{{Path: "src/parser.rs", Type: types.DiffLineFileHeader}},
+			{{Path: "src/parser.rs", Type: types.DiffLineContext, NewLine: 10}},
+			{{Path: "src/parser.rs", Type: types.DiffLineAdded, NewLine: 11}},
+			{{Path: "src/parser.rs", Type: types.DiffLineAdded, NewLine: 12}},
+			nil,
+			{{Path: "src/parser.rs", Type: types.DiffLineContext, NewLine: 40}},
+		}, parseDiffLineIdentities(contents))
+	})
+
+	t.Run("a row with two records shows both of their lines", func(t *testing.T) {
+		contents := []gocui.DiffLineContent{
+			row("two │ TWO", "1;d;2;2;file1", "1;a;2;;file1"),
+		}
+
+		assert.Equal(t, [][]parsedDiffLine{
+			{
+				{Path: "file1", Type: types.DiffLineDeleted, NewLine: 2, OldLine: 2},
+				{Path: "file1", Type: types.DiffLineAdded, NewLine: 2},
+			},
+		}, parseDiffLineIdentities(contents))
+	})
+
+	t.Run("a rendering without records is parsed as a diff", func(t *testing.T) {
+		bufferLines := strings.Split(twoFileDiff, "\n")
+		contents := make([]gocui.DiffLineContent, len(bufferLines))
+		for i, line := range bufferLines {
+			contents[i] = row(line)
+		}
+
+		identities := parseDiffLineIdentities(contents)
+		for i, parsed := range parseAllDiffLinesFromBuffer(bufferLines) {
+			assert.True(t, parsed.ok, "line %d", i)
+			assert.Equal(t, []parsedDiffLine{parsed.parsed}, identities[i], "line %d", i)
+		}
+	})
+}

pkg/gui/controllers/helpers/diff_line_restore.go

@@ -0,0 +1,312 @@
+package helpers
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/gocui"
+	"github.com/jesseduffield/lazygit/pkg/gui/types"
+	"github.com/jesseduffield/lazygit/pkg/tasks"
+	"github.com/samber/lo"
+)
+
+// Keeping a diff view where it is when the same diff is rendered again differently.
+// The line the user is on is remembered by identity (diff_line_helper.go), because a
+// new rendering puts it on a different line of the view — and may not have it at all,
+// which is what the fallbacks below are for.
+
+// diffLineAnchor is a line for a restore to land on: the identity to find it by in
+// the new rendering, and the screen row it was on, so that it can be put back there.
+type diffLineAnchor struct {
+	identity types.DiffLineInfo
+	row      int
+}
+
+// PreserveDiffPositionOnRerender remembers where a diff view is and puts it back
+// there as it next re-renders, instead of leaving the user at the top of a new
+// rendering of the diff they were already reading. Call it on the view about to be
+// re-rendered, right before triggering the re-render — on both panes of the main
+// window where both are being rendered again, since either of them may hold the diff
+// being read; a pane that isn't showing is left alone.
+//
+// The line to keep is the selected one, and the middle visible line when there is no
+// selection or it has been scrolled out of sight — what the user is looking at, rather
+// than the view's top edge or a selection they have long since left behind. It may not
+// survive the re-render: a context line goes when the context size shrinks, and a
+// whole hunk or file goes when whitespace stops counting. So the lines around it come
+// along as fallbacks and the view lands on the nearest one that is still there, put
+// back on the screen row it was on. With none of them left — and with a renderer that
+// says nothing about its rows there is nothing to look for in the first place — the
+// view keeps the scroll offset it had, which is still nearer to what was being read
+// than the top of the diff.
+//
+// An off-screen selection is still put back on the diff line it was on, wherever the
+// new rendering has that; it is only the view that stays where it is.
+func (self *DiffLineHelper) PreserveDiffPositionOnRerender(view *gocui.View) {
+	// A view that isn't the one its window is currently showing — the merge-conflicts
+	// view takes the main window over — isn't the one about to be re-rendered, so a
+	// restore installed on it would sit there and claim a later render instead.
+	if !view.Visible {
+		return
+	}
+
+	// The re-render is produced by a different command from the one behind what is on
+	// screen — another context size, another renderer — so without being told otherwise
+	// it would be taken for content the user has never seen and shown from the top.
+	// Whether or not a line of the old rendering can be found in the new one, the offset
+	// into it is nearer to where they were reading than the top is.
+	if manager := self.c.GetViewBufferManagerForView(view); manager != nil {
+		manager.SetKeepScrollPositionForNextTask()
+	}
+
+	showSelection := view.Highlight
+	selectionOnScreen := showSelection && view.IsLineVisible(view.SelectedLineIdx())
+	anchorViewLine := view.MiddleVisibleLineIdx()
+	if selectionOnScreen {
+		anchorViewLine = view.SelectedLineIdx()
+	}
+	// A selection that has been scrolled away from is put back by its own lines rather
+	// than by the anchor's, so that it comes out on the same line of the diff without
+	// the view having to go there.
+	var selectionCandidates []diffLineAnchor
+	if showSelection && !selectionOnScreen {
+		selectionCandidates = self.nearbyDiffLines(view, view.SelectedLineIdx())
+	}
+
+	self.restoreDiffLinePositionOnRerender(view, self.nearbyDiffLines(view, anchorViewLine),
+		func(anchor diffLineAnchor, viewLine int) {
+			// Put the line back on the screen row it was on, clamped into the view for
+			// the fallback lines, which can come from off screen.
+			row := lo.Clamp(anchor.row, 0, max(0, view.InnerHeight()-1))
+			view.SetOrigin(0, max(0, viewLine-row))
+			if showSelection {
+				// A range's other end is a view line, which the new rendering has made
+				// mean something else, so the selection collapses to the line we landed
+				// on. The origin is already where it should be, so moving the cursor
+				// there mustn't scroll.
+				view.CancelRangeSelect()
+				view.FocusPoint(0, self.selectionLine(view, selectionCandidates, viewLine), false)
+			}
+		})
+}
+
+// selectionLine returns the line to put the cursor on once a re-render is on screen:
+// the line the position anchor landed on, which is the selected one whenever it was
+// on screen, and otherwise the nearest surviving line to where the selection was —
+// found among its own candidates, since the anchor's are a search of the diff from
+// somewhere else entirely.
+func (self *DiffLineHelper) selectionLine(
+	view *gocui.View, candidates []diffLineAnchor, anchorViewLine int,
+) int {
+	if len(candidates) == 0 {
+		return anchorViewLine
+	}
+	_, bufferLine := self.nearestSurvivingCandidate(view.DiffLineContents(), candidates)
+	if bufferLine == -1 {
+		return anchorViewLine
+	}
+	if viewLine, ok := view.ViewLineForBufferLine(bufferLine); ok {
+		return viewLine
+	}
+	return anchorViewLine
+}
+
+// restoreDiffLinePositionOnRerender arranges for view's next re-render to land on the
+// first of the given candidate lines the new rendering still has, calling place with
+// that candidate and the view line it ended up on. The candidates are in priority
+// order (see nearbyDiffLines); if the rendering has none of them, place isn't called
+// and the view re-renders as it otherwise would.
+//
+// The nearest candidate is looked for as the content loads, so that the re-render can
+// be revealed at the right position as soon as that line and a screenful below it
+// have arrived. Only the nearest one, because the candidates aren't in load order: a
+// farther one can load first, and landing on it while a nearer one is still on its
+// way would be settling for worse. The rest are considered together once the whole
+// rendering is there.
+func (self *DiffLineHelper) restoreDiffLinePositionOnRerender(
+	view *gocui.View, candidates []diffLineAnchor, place func(anchor diffLineAnchor, viewLine int),
+) {
+	manager := self.c.GetViewBufferManagerForView(view)
+	if manager == nil || len(candidates) == 0 {
+		return
+	}
+
+	// The readiness check below runs on the task's own goroutine, where neither the
+	// view's dimensions nor the repo we are in may be read — a repo switch replaces
+	// the latter — so take both here, on the UI thread, for it to work from.
+	viewHeight := view.InnerHeight()
+	worktreePath := self.c.Git().RepoPaths.WorktreePath()
+
+	// What the search of the loading content has found, and how far it has looked, so
+	// that each line is looked at once.
+	found := diffLineAnchor{}
+	foundLine := -1
+	scanned := 0
+
+	manager.SetRestoreForNextTask(&tasks.RenderRestore{
+		FirstPaintReady: func() bool {
+			if foundLine == -1 {
+				rows := view.OffscreenDiffLineContentsFrom(scanned)
+				for i, row := range rows {
+					if rowShowsDiffLine(row, worktreePath, candidates[0].identity) {
+						found, foundLine = candidates[0], scanned+i
+						break
+					}
+				}
+				scanned += len(rows)
+				if foundLine == -1 {
+					return false
+				}
+			}
+			// Wait for a screenful below the line as well, so that the re-render isn't
+			// revealed with it stranded at the bottom of a half-filled view.
+			return view.OffscreenLineCount() >= foundLine+viewHeight
+		},
+		Apply: func(swapIn func()) bool {
+			anchor, bufferLine := found, foundLine
+			if bufferLine == -1 {
+				anchor, bufferLine = self.nearestSurvivingCandidate(view.OffscreenDiffLineContents(), candidates)
+			}
+
+			swapIn()
+
+			if bufferLine == -1 {
+				return false
+			}
+			viewLine, ok := view.ViewLineForBufferLine(bufferLine)
+			if !ok {
+				return false
+			}
+			place(anchor, viewLine)
+			return true
+		},
+	})
+}
+
+// nearbyDiffLines collects the lines of view's rendered diff as candidates for a
+// restore to land on, ordered by proximity to the anchor line — the anchor itself
+// first, then outward, preferring at-or-below on ties — each tagged with the screen
+// row it is on. A restore lands on the first of them its re-render still has, so this
+// order makes it land as near as possible to where the user was.
+//
+// The walk covers the whole diff rather than stopping at the change lines on either
+// side of the anchor, which a context-size change always keeps: ignoring whitespace
+// keeps nothing in particular, and can take a hunk or a whole file out of the diff,
+// leaving the nearest surviving line in a neighbouring file.
+func (self *DiffLineHelper) nearbyDiffLines(view *gocui.View, anchorViewLine int) []diffLineAnchor {
+	anchor, ok := view.BufferLineForViewLine(anchorViewLine)
+	if !ok {
+		return nil
+	}
+	resolved := self.resolveDiffLines(view.DiffLineContents())
+	if anchor >= len(resolved) {
+		return nil
+	}
+	rows := screenRows(view, len(resolved))
+
+	candidates := make([]diffLineAnchor, 0, len(resolved))
+	collect := func(bufferLine int) {
+		if line := resolved[bufferLine]; line.ok {
+			candidates = append(candidates, diffLineAnchor{identity: line.info, row: rows[bufferLine]})
+		}
+	}
+	collect(anchor)
+	for below, above := anchor+1, anchor-1; below < len(resolved) || above >= 0; below, above = below+1, above-1 {
+		if below < len(resolved) {
+			collect(below)
+		}
+		if above >= 0 {
+			collect(above)
+		}
+	}
+	return candidates
+}
+
+// screenRows maps each line of view's content to the screen row it is drawn on. The
+// lines above the visible ones get -1 and those below them the view's height, so that
+// putting one of them back where it was lands it at the top or bottom edge.
+func screenRows(view *gocui.View, bufferLineCount int) []int {
+	height := view.InnerHeight()
+	originY := view.OriginY()
+
+	rows := make([]int, bufferLineCount)
+	for i := range rows {
+		rows[i] = -1
+	}
+	lastVisible := -1
+	for y := originY; y < min(originY+height, view.ViewLinesHeight()); y++ {
+		bufferLine, ok := view.BufferLineForViewLine(y)
+		if !ok || bufferLine >= bufferLineCount {
+			continue
+		}
+		if rows[bufferLine] == -1 {
+			rows[bufferLine] = y - originY
+		}
+		lastVisible = bufferLine
+	}
+	for i := lastVisible + 1; i < bufferLineCount; i++ {
+		rows[i] = height
+	}
+	return rows
+}
+
+// nearestSurvivingCandidate returns the first of the candidates that the given
+// rendering still shows, and the line of it that does. The rendering is indexed
+// first, rather than searched once per candidate: the candidate list is as long as
+// the diff, and so is the rendering.
+func (self *DiffLineHelper) nearestSurvivingCandidate(
+	contents []gocui.DiffLineContent, candidates []diffLineAnchor,
+) (diffLineAnchor, int) {
+	lines := map[patchLine]int{}
+	for i, identities := range self.resolveDiffLineIdentities(contents) {
+		for _, identity := range identities {
+			if _, seen := lines[patchLineOf(identity)]; !seen {
+				lines[patchLineOf(identity)] = i
+			}
+		}
+	}
+
+	for _, candidate := range candidates {
+		if line, ok := lines[patchLineOf(candidate.identity)]; ok {
+			return candidate, line
+		}
+	}
+	return diffLineAnchor{}, -1
+}
+
+// rowShowsDiffLine reports whether the given row of a rendering shows the given diff
+// line — among any others it shows, since a side-by-side rendering puts a deletion
+// beside the addition replacing it. It only knows what the renderer states about the
+// row, since the alternative, parsing the rendering as a diff, needs whole hunks and
+// this is asked of content that is still loading. It takes the repo's worktree path
+// rather than reading it, being asked off the UI thread.
+func rowShowsDiffLine(row gocui.DiffLineContent, worktreePath string, target types.DiffLineInfo) bool {
+	return lo.SomeBy(row.Metadata, func(record string) bool {
+		parsed, ok := parseDiffLineMetadata(record)
+		return ok && patchLineOf(diffLineInfoIn(worktreePath, parsed)) == patchLineOf(target)
+	})
+}
+
+// patchLine records what stays the same about a diff line when the same diff is
+// rendered again differently: which file it belongs to, the line number that
+// identifies it on the side it belongs to, and what kind of line it is.
+type patchLine struct {
+	path string
+	// Every kind of content line collapses into DiffLineContext, since an addition
+	// and the context line it turns into when whitespace stops counting are the same
+	// line of the same file. The header rows keep their kind: a file's header and the
+	// first line of the file it heads are not the same place.
+	kind types.DiffLineType
+	// The old file's line number for a deletion, since two consecutive deletions
+	// share a new-file position and differ only here; the new file's otherwise.
+	line       int
+	isDeletion bool
+}
+
+func patchLineOf(info types.DiffLineInfo) patchLine {
+	switch info.Type {
+	case types.DiffLineFileHeader, types.DiffLineHunkHeader:
+		return patchLine{path: info.Path, kind: info.Type, line: info.NewLine}
+	case types.DiffLineDeleted:
+		return patchLine{path: info.Path, kind: types.DiffLineContext, line: info.OldLine, isDeletion: true}
+	default:
+		return patchLine{path: info.Path, kind: types.DiffLineContext, line: info.NewLine}
+	}
+}

pkg/integration/tests/main_view/keep_position_in_both_panes_when_changing_context_size.go

@@ -0,0 +1,71 @@
+package main_view
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var KeepPositionInBothPanesWhenChangingContextSize = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Changing the diff's context size keeps the place in the lower pane too, not only in the upper one",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	Width:        120,
+	Height:       30,
+	SetupConfig: func(cfg *config.AppConfig) {
+		cfg.GetUserConfig().Gui.UseHunkModeInStagingView = false
+	},
+	SetupRepo: func(shell *Shell) {
+		lines := make([]string, 40)
+		for i := range lines {
+			lines[i] = fmt.Sprintf("line%02d", i+1)
+		}
+		shell.CreateFileAndAdd("file1", strings.Join(lines, "\n")+"\n")
+		shell.Commit("one")
+
+		// Four staged changes, far enough apart that they stay four hunks as the
+		// context size grows, and one unstaged one to split the file's diff across
+		// both panes.
+		for _, i := range []int{5, 15, 25, 35} {
+			lines[i-1] = strings.ToUpper(lines[i-1])
+		}
+		shell.UpdateFile("file1", strings.Join(lines, "\n")+"\n")
+		shell.GitAddAll()
+
+		lines[39] = strings.ToUpper(lines[39])
+		shell.UpdateFile("file1", strings.Join(lines, "\n")+"\n")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Files().
+			IsFocused().
+			Press(keys.Universal.FocusMainView)
+
+		// The lower pane holds the staged changes; getting to the last of them scrolls
+		// it, so there is a position to lose.
+		t.Views().Main().
+			IsFocused().
+			PressTab()
+
+		t.Views().Secondary().
+			IsFocused().
+			Press(keys.Main.NextHunk).
+			Press(keys.Main.NextHunk).
+			Press(keys.Main.NextHunk).
+			SelectedLines(
+				Contains("-line35"),
+			).
+			SelectedLineIdx(35).
+			OriginY(14).
+			Press(keys.Universal.IncreaseContextInDiffView).
+			Tap(func() {
+				t.ExpectToast(Equals("Changed diff context size to 4"))
+			}).
+			SelectedLines(
+				Contains("-line35"),
+			).
+			SelectedLineIdx(42).
+			OriginY(21)
+	},
+})

pkg/integration/tests/main_view/keep_position_when_changing_context_size.go

@@ -0,0 +1,97 @@
+package main_view
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var KeepPositionWhenChangingContextSize = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Changing the diff's context size keeps the line you were looking at where it was",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	Width:        120,
+	Height:       30,
+	SetupConfig: func(cfg *config.AppConfig) {
+		cfg.GetUserConfig().Gui.UseHunkModeInStagingView = false
+	},
+	SetupRepo: func(shell *Shell) {
+		lines := make([]string, 40)
+		for i := range lines {
+			lines[i] = fmt.Sprintf("line%02d", i+1)
+		}
+		shell.CreateFileAndAdd("file1", strings.Join(lines, "\n")+"\n")
+		shell.Commit("one")
+
+		// Four changes, far enough apart that they stay four hunks as the context
+		// size grows.
+		for _, i := range []int{5, 15, 25, 35} {
+			lines[i-1] = strings.ToUpper(lines[i-1])
+		}
+		shell.UpdateFile("file1", strings.Join(lines, "\n")+"\n")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Files().
+			IsFocused().
+			Press(keys.Universal.FocusMainView)
+
+		t.Views().Main().
+			IsFocused().
+			Press(keys.Main.NextHunk).
+			Press(keys.Main.NextHunk).
+			Press(keys.Main.NextHunk).
+			SelectedLines(
+				Contains("-line35"),
+			).
+			// The diff is longer than the view, so getting to the last hunk scrolled
+			// it: the selected line sits 21 rows down the screen.
+			SelectedLineIdx(35).
+			OriginY(14).
+			Press(keys.Universal.IncreaseContextInDiffView).
+			Tap(func() {
+				t.ExpectToast(Equals("Changed diff context size to 4"))
+			}).
+			// A context line more on either side of each of the four hunks pushes the
+			// selected line seven lines further into the diff. The view follows it, so
+			// it is still the same line on the same screen row (42 - 21 = 21).
+			SelectedLines(
+				Contains("-line35"),
+			).
+			SelectedLineIdx(42).
+			OriginY(21).
+			Press(keys.Universal.DecreaseContextInDiffView).
+			Tap(func() {
+				t.ExpectToast(Equals("Changed diff context size to 3"))
+			}).
+			Press(keys.Universal.DecreaseContextInDiffView).
+			Tap(func() {
+				t.ExpectToast(Equals("Changed diff context size to 2"))
+			}).
+			// And the same the other way (28 - 21 = 7).
+			SelectedLines(
+				Contains("-line35"),
+			).
+			SelectedLineIdx(28).
+			OriginY(7).
+			// Leaving the view gives up the selection but not the scroll position, and
+			// with no selection to keep, it is the middle visible line that stays put.
+			PressEscape()
+
+		t.Views().Files().
+			IsFocused().
+			Press(keys.Universal.IncreaseContextInDiffView).
+			Tap(func() {
+				t.ExpectToast(Equals("Changed diff context size to 3"))
+			})
+
+		// The middle visible line here is a hunk's header, and a context-size change
+		// rewrites those — they name the lines the hunk covers. So the restore falls
+		// back to the nearest line that does survive, the context line just below it,
+		// and puts that back on the row it was on.
+		t.Views().Main().
+			SelectionIsHidden().
+			OriginY(12)
+	},
+})

pkg/integration/tests/main_view/keep_position_when_the_selection_is_off_screen.go

@@ -0,0 +1,65 @@
+package main_view
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var KeepPositionWhenTheSelectionIsOffScreen = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "A re-render keeps the lines that are on screen where they are, not a selection scrolled away from",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	Width:        120,
+	Height:       30,
+	SetupConfig: func(cfg *config.AppConfig) {
+		cfg.GetUserConfig().Gui.UseHunkModeInStagingView = false
+		// Half a diff per scroll, to leave the selection well behind in two presses.
+		cfg.GetUserConfig().Gui.ScrollHeight = 15
+	},
+	SetupRepo: func(shell *Shell) {
+		lines := make([]string, 60)
+		for i := range lines {
+			lines[i] = fmt.Sprintf("line%02d", i+1)
+		}
+		shell.CreateFileAndAdd("file1", strings.Join(lines, "\n")+"\n")
+		shell.Commit("one")
+
+		for _, i := range []int{5, 15, 25, 35, 45, 55} {
+			lines[i-1] = strings.ToUpper(lines[i-1])
+		}
+		shell.UpdateFile("file1", strings.Join(lines, "\n")+"\n")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Files().
+			IsFocused().
+			Press(keys.Universal.FocusMainView)
+
+		t.Views().Main().
+			IsFocused().
+			SelectedLines(
+				Contains("-line05"),
+			).
+			// Reading on past the selection leaves it far behind, off the top of the
+			// view.
+			Press(keys.Universal.ScrollDownMain).
+			Press(keys.Universal.ScrollDownMain).
+			OriginY(30).
+			Press(keys.Universal.DecreaseContextInDiffView).
+			Tap(func() {
+				t.ExpectToast(Equals("Changed diff context size to 2"))
+			}).
+			// A context line less on either side of the four hunks above what is on
+			// screen pulls it nine lines up the diff, and the view follows it there: the
+			// lines the user was reading are still on the rows they were on.
+			OriginY(21).
+			// The selection is where it always was, on its own line of the diff, rather
+			// than having been dragged back into view.
+			SelectedLines(
+				Contains("-line05"),
+			).
+			SelectedLineIdx(7)
+	},
+})

pkg/integration/tests/main_view/keep_scroll_when_the_diff_cant_be_read.go

@@ -0,0 +1,58 @@
+package main_view
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var KeepScrollWhenTheDiffCantBeRead = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Changing the context size under a diff renderer whose rows can't be placed keeps the scroll position rather than jumping to the top",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	Width:        120,
+	Height:       30,
+	SetupConfig: func(cfg *config.AppConfig) {
+		cfg.GetUserConfig().Gui.UseHunkModeInStagingView = false
+		// A renderer that says nothing about which line of which file each row shows,
+		// and mangles the diff enough that it can't be read back as one either: no line
+		// of it can be looked for in the re-render.
+		cfg.GetUserConfig().Git.DiffRenderers = []config.DiffRendererConfig{
+			{Name: "opaque", Command: `sed -e 's/^/| /'`},
+		}
+	},
+	SetupRepo: func(shell *Shell) {
+		lines := make([]string, 40)
+		for i := range lines {
+			lines[i] = fmt.Sprintf("line%02d", i+1)
+		}
+		shell.CreateFileAndAdd("file1", strings.Join(lines, "\n")+"\n")
+		shell.Commit("one")
+
+		for _, i := range []int{5, 15, 25, 35} {
+			lines[i-1] = strings.ToUpper(lines[i-1])
+		}
+		shell.UpdateFile("file1", strings.Join(lines, "\n")+"\n")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Files().
+			IsFocused().
+			Press(keys.Universal.ScrollDownMain).
+			Press(keys.Universal.ScrollDownMain).
+			Press(keys.Universal.ScrollDownMain)
+
+		t.Views().Main().
+			Content(Contains("| +LINE05")).
+			OriginY(6).
+			Tap(func() {
+				t.Views().Files().Press(keys.Universal.IncreaseContextInDiffView)
+				t.ExpectToast(Equals("Changed diff context size to 4"))
+			}).
+			// The re-render is a different command, and nothing in its output can be
+			// matched up with what was on screen, so the offset is all there is to keep —
+			// and it is a good deal closer than the top.
+			OriginY(6)
+	},
+})

pkg/integration/tests/test_list.go

@@ -371,6 +371,10 @@ var tests = []*components.IntegrationTest{
 	main_view.EscapeDismissesSelection,
 	main_view.FileNavigationScrollsToTheTop,
 	main_view.HideSelectionWhenChangesVanish,
+	main_view.KeepPositionInBothPanesWhenChangingContextSize,
+	main_view.KeepPositionWhenChangingContextSize,
+	main_view.KeepPositionWhenTheSelectionIsOffScreen,
+	main_view.KeepScrollWhenTheDiffCantBeRead,
 	main_view.KeepSelectionVisibleWhenDiffShrinks,
 	main_view.NavigateByHunkAndFile,
 	main_view.NoSelectionOverABinaryDiff,