diff --git a/routers/web/feed/profile_test.go b/routers/web/feed/profile_test.go
index a0f1509269..bf9492b57b 100644
--- a/routers/web/feed/profile_test.go
+++ b/routers/web/feed/profile_test.go
@@ -23,7 +23,6 @@ func TestCheckGetOrgFeedsAsOrgMember(t *testing.T) {
 		ctx, resp := contexttest.MockContext(t, "org3.atom")
 		ctx.ContextUser = unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 3})
 		contexttest.LoadUser(t, ctx, 2)
-		ctx.IsSigned = true
 		feed.ShowUserFeedAtom(ctx)
 		assert.Contains(t, resp.Body.String(), "<entry>") // Should contain 1 private entry
 	})
@@ -31,7 +30,6 @@ func TestCheckGetOrgFeedsAsOrgMember(t *testing.T) {
 		ctx, resp := contexttest.MockContext(t, "org3.atom")
 		ctx.ContextUser = unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 3})
 		contexttest.LoadUser(t, ctx, 5)
-		ctx.IsSigned = true
 		feed.ShowUserFeedAtom(ctx)
 		assert.NotContains(t, resp.Body.String(), "<entry>") // Should not contain any entries
 	})
diff --git a/routers/web/repo/commit.go b/routers/web/repo/commit.go
index f702b2de16..27f5651ecb 100644
--- a/routers/web/repo/commit.go
+++ b/routers/web/repo/commit.go
@@ -279,7 +279,7 @@ func Diff(ctx *context.Context) {
 
 	diffBlobExcerptData := &gitdiff.DiffBlobExcerptData{
 		BaseLink:      ctx.Repo.RepoLink + "/blob_excerpt",
-		DiffStyle:     ctx.FormString("style"),
+		DiffStyle:     GetDiffViewStyle(ctx),
 		AfterCommitID: commitID,
 	}
 	gitRepo := ctx.Repo.GitRepo
diff --git a/routers/web/repo/compare.go b/routers/web/repo/compare.go
index 6ccf0901b9..150a8583c8 100644
--- a/routers/web/repo/compare.go
+++ b/routers/web/repo/compare.go
@@ -499,7 +499,7 @@ func PrepareCompareDiff(
 	ctx.Data["Diff"] = diff
 	ctx.Data["DiffBlobExcerptData"] = &gitdiff.DiffBlobExcerptData{
 		BaseLink:      ci.HeadRepo.Link() + "/blob_excerpt",
-		DiffStyle:     ctx.FormString("style"),
+		DiffStyle:     GetDiffViewStyle(ctx),
 		AfterCommitID: headCommitID,
 	}
 	ctx.Data["DiffNotAvailable"] = diffShortStat.NumFiles == 0
@@ -756,7 +756,7 @@ func ExcerptBlob(ctx *context.Context) {
 
 	diffBlobExcerptData := &gitdiff.DiffBlobExcerptData{
 		BaseLink:      ctx.Repo.RepoLink + "/blob_excerpt",
-		DiffStyle:     ctx.FormString("style"),
+		DiffStyle:     GetDiffViewStyle(ctx),
 		AfterCommitID: commitID,
 	}
 
diff --git a/routers/web/repo/middlewares.go b/routers/web/repo/middlewares.go
index 7518e6feae..c7c9da498b 100644
--- a/routers/web/repo/middlewares.go
+++ b/routers/web/repo/middlewares.go
@@ -7,8 +7,11 @@ import (
 	"strconv"
 
 	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/optional"
+	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/services/context"
+	"code.gitea.io/gitea/services/gitdiff"
 	user_service "code.gitea.io/gitea/services/user"
 )
 
@@ -28,36 +31,24 @@ func SetEditorconfigIfExists(ctx *context.Context) {
 	ctx.Data["Editorconfig"] = ec
 }
 
+func GetDiffViewStyle(ctx *context.Context) string {
+	return util.Iif(ctx.Data["IsSplitStyle"] == true, gitdiff.DiffStyleSplit, gitdiff.DiffStyleUnified)
+}
+
 // SetDiffViewStyle set diff style as render variable
 func SetDiffViewStyle(ctx *context.Context) {
-	queryStyle := ctx.FormString("style")
-
-	if !ctx.IsSigned {
-		ctx.Data["IsSplitStyle"] = queryStyle == "split"
-		return
+	style := ctx.FormString("style")
+	if ctx.IsSigned {
+		style = util.IfZero(style, ctx.Doer.DiffViewStyle)
+		style = util.Iif(style == gitdiff.DiffStyleSplit, gitdiff.DiffStyleSplit, gitdiff.DiffStyleUnified)
+		if style != ctx.Doer.DiffViewStyle {
+			err := user_service.UpdateUser(ctx, ctx.Doer, &user_service.UpdateOptions{DiffViewStyle: optional.Some(style)})
+			if err != nil {
+				log.Error("UpdateUser DiffViewStyle: %v", err)
+			}
+		}
 	}
-
-	var (
-		userStyle = ctx.Doer.DiffViewStyle
-		style     string
-	)
-
-	if queryStyle == "unified" || queryStyle == "split" {
-		style = queryStyle
-	} else if userStyle == "unified" || userStyle == "split" {
-		style = userStyle
-	} else {
-		style = "unified"
-	}
-
 	ctx.Data["IsSplitStyle"] = style == "split"
-
-	opts := &user_service.UpdateOptions{
-		DiffViewStyle: optional.Some(style),
-	}
-	if err := user_service.UpdateUser(ctx, ctx.Doer, opts); err != nil {
-		ctx.ServerError("UpdateUser", err)
-	}
 }
 
 // SetWhitespaceBehavior set whitespace behavior as render variable
diff --git a/routers/web/repo/middlewares_test.go b/routers/web/repo/middlewares_test.go
new file mode 100644
index 0000000000..c6dc2e4615
--- /dev/null
+++ b/routers/web/repo/middlewares_test.go
@@ -0,0 +1,59 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repo
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/services/contexttest"
+	"code.gitea.io/gitea/services/gitdiff"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDiffViewStyle(t *testing.T) {
+	unittest.PrepareTestEnv(t)
+
+	t.Run("AnonymousUser", func(t *testing.T) {
+		ctx, _ := contexttest.MockContext(t, "/any")
+		SetDiffViewStyle(ctx)
+		assert.Equal(t, gitdiff.DiffStyleUnified, GetDiffViewStyle(ctx))
+
+		ctx, _ = contexttest.MockContext(t, "/any?style=split")
+		SetDiffViewStyle(ctx)
+		assert.Equal(t, gitdiff.DiffStyleSplit, GetDiffViewStyle(ctx))
+
+		ctx, _ = contexttest.MockContext(t, "/any")
+		SetDiffViewStyle(ctx)
+		assert.Equal(t, gitdiff.DiffStyleUnified, GetDiffViewStyle(ctx)) // at the moment, anonymous users don't have a saved preference
+	})
+
+	t.Run("SignedInUser", func(t *testing.T) {
+		ctx, _ := contexttest.MockContext(t, "/any")
+		contexttest.LoadUser(t, ctx, 2)
+		SetDiffViewStyle(ctx)
+		assert.Equal(t, gitdiff.DiffStyleUnified, GetDiffViewStyle(ctx))
+
+		ctx, _ = contexttest.MockContext(t, "/any?style=split")
+		contexttest.LoadUser(t, ctx, 2)
+		SetDiffViewStyle(ctx)
+		assert.Equal(t, gitdiff.DiffStyleSplit, GetDiffViewStyle(ctx))
+
+		ctx, _ = contexttest.MockContext(t, "/any")
+		contexttest.LoadUser(t, ctx, 2)
+		SetDiffViewStyle(ctx)
+		assert.Equal(t, gitdiff.DiffStyleSplit, GetDiffViewStyle(ctx))
+
+		ctx, _ = contexttest.MockContext(t, "/any?style=unified")
+		contexttest.LoadUser(t, ctx, 2)
+		SetDiffViewStyle(ctx)
+		assert.Equal(t, gitdiff.DiffStyleUnified, GetDiffViewStyle(ctx))
+
+		ctx, _ = contexttest.MockContext(t, "/any")
+		contexttest.LoadUser(t, ctx, 2)
+		SetDiffViewStyle(ctx)
+		assert.Equal(t, gitdiff.DiffStyleUnified, GetDiffViewStyle(ctx))
+	})
+}
diff --git a/routers/web/repo/pull.go b/routers/web/repo/pull.go
index 7c67d614f6..ecc1bb0644 100644
--- a/routers/web/repo/pull.go
+++ b/routers/web/repo/pull.go
@@ -865,7 +865,7 @@ func viewPullFiles(ctx *context.Context, beforeCommitID, afterCommitID string) {
 	ctx.Data["DiffBlobExcerptData"] = &gitdiff.DiffBlobExcerptData{
 		BaseLink:       ctx.Repo.RepoLink + "/blob_excerpt",
 		PullIssueIndex: pull.Index,
-		DiffStyle:      ctx.FormString("style"),
+		DiffStyle:      GetDiffViewStyle(ctx),
 		AfterCommitID:  afterCommitID,
 	}
 	ctx.Data["DiffNotAvailable"] = diffShortStat.NumFiles == 0
diff --git a/services/contexttest/context_tests.go b/services/contexttest/context_tests.go
index 3a072e089a..33e632ea4d 100644
--- a/services/contexttest/context_tests.go
+++ b/services/contexttest/context_tests.go
@@ -162,8 +162,10 @@ func LoadUser(t *testing.T, ctx gocontext.Context, userID int64) {
 	switch ctx := ctx.(type) {
 	case *context.Context:
 		ctx.Doer = doer
+		ctx.IsSigned = true
 	case *context.APIContext:
 		ctx.Doer = doer
+		ctx.IsSigned = true
 	default:
 		assert.FailNow(t, "context is not *context.Context or *context.APIContext")
 	}
diff --git a/services/gitdiff/gitdiff.go b/services/gitdiff/gitdiff.go
index 17eb3d4280..be5c1dbece 100644
--- a/services/gitdiff/gitdiff.go
+++ b/services/gitdiff/gitdiff.go
@@ -200,6 +200,11 @@ type DiffBlobExcerptData struct {
 	AfterCommitID  string
 }
 
+const (
+	DiffStyleSplit   = "split"
+	DiffStyleUnified = "unified"
+)
+
 func (d *DiffLine) RenderBlobExcerptButtons(fileNameHash string, data *DiffBlobExcerptData) template.HTML {
 	dataHiddenCommentIDs := strings.Join(base.Int64sToStrings(d.SectionInfo.HiddenCommentIDs), ",")
 	anchor := fmt.Sprintf("diff-%sK%d", fileNameHash, d.SectionInfo.RightIdx)