Allow to mark files in a PR as viewed (#19007)

Users can now mark files in PRs as viewed, resulting in them not being shown again by default when they reopen the PR again.

16 files changed, 493 insertions, 45 deletions

diff --git a/models/migrations/migrations.go b/models/migrations/migrations.go
index 817ba3bfac..5a2297ac0d 100644
--- a/models/migrations/migrations.go
+++ b/models/migrations/migrations.go
@@ -385,6 +385,8 @@ var migrations = []Migration{
 	NewMigration("Add allow edits from maintainers to PullRequest table", addAllowMaintainerEdit),
 	// v214 -> v215
 	NewMigration("Add auto merge table", addAutoMergeTable),
+	// v215 -> v216
+	NewMigration("allow to view files in PRs", addReviewViewedFiles),
 }
 
 // GetCurrentDBVersion returns the current db version
diff --git a/models/migrations/v215.go b/models/migrations/v215.go
new file mode 100644
index 0000000000..138917edbe
--- /dev/null
+++ b/models/migrations/v215.go
@@ -0,0 +1,25 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package migrations
+
+import (
+	"code.gitea.io/gitea/models/pull"
+	"code.gitea.io/gitea/modules/timeutil"
+
+	"xorm.io/xorm"
+)
+
+func addReviewViewedFiles(x *xorm.Engine) error {
+	type ReviewState struct {
+		ID           int64                       `xorm:"pk autoincr"`
+		UserID       int64                       `xorm:"NOT NULL UNIQUE(pull_commit_user)"`
+		PullID       int64                       `xorm:"NOT NULL UNIQUE(pull_commit_user) DEFAULT 0"`
+		CommitSHA    string                      `xorm:"NOT NULL VARCHAR(40) UNIQUE(pull_commit_user)"`
+		UpdatedFiles map[string]pull.ViewedState `xorm:"NOT NULL LONGTEXT JSON"`
+		UpdatedUnix  timeutil.TimeStamp          `xorm:"updated"`
+	}
+
+	return x.Sync2(new(ReviewState))
+}
diff --git a/models/pull/review_state.go b/models/pull/review_state.go
new file mode 100644
index 0000000000..59a03c20e8
--- /dev/null
+++ b/models/pull/review_state.go
@@ -0,0 +1,139 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+package pull
+
+import (
+	"context"
+	"fmt"
+
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/timeutil"
+)
+
+// ViewedState stores for a file in which state it is currently viewed
+type ViewedState uint8
+
+const (
+	Unviewed   ViewedState = iota
+	HasChanged             // cannot be set from the UI/ API, only internally
+	Viewed
+)
+
+func (viewedState ViewedState) String() string {
+	switch viewedState {
+	case Unviewed:
+		return "unviewed"
+	case HasChanged:
+		return "has-changed"
+	case Viewed:
+		return "viewed"
+	default:
+		return fmt.Sprintf("unknown(value=%d)", viewedState)
+	}
+}
+
+// ReviewState stores for a user-PR-commit combination which files the user has already viewed
+type ReviewState struct {
+	ID           int64                  `xorm:"pk autoincr"`
+	UserID       int64                  `xorm:"NOT NULL UNIQUE(pull_commit_user)"`
+	PullID       int64                  `xorm:"NOT NULL UNIQUE(pull_commit_user) DEFAULT 0"`   // Which PR was the review on?
+	CommitSHA    string                 `xorm:"NOT NULL VARCHAR(40) UNIQUE(pull_commit_user)"` // Which commit was the head commit for the review?
+	UpdatedFiles map[string]ViewedState `xorm:"NOT NULL LONGTEXT JSON"`                        // Stores for each of the changed files of a PR whether they have been viewed, changed since last viewed, or not viewed
+	UpdatedUnix  timeutil.TimeStamp     `xorm:"updated"`                                       // Is an accurate indicator of the order of commits as we do not expect it to be possible to make reviews on previous commits
+}
+
+func init() {
+	db.RegisterModel(new(ReviewState))
+}
+
+// GetReviewState returns the ReviewState with all given values prefilled, whether or not it exists in the database.
+// If the review didn't exist before in the database, it won't afterwards either.
+// The returned boolean shows whether the review exists in the database
+func GetReviewState(ctx context.Context, userID, pullID int64, commitSHA string) (*ReviewState, bool, error) {
+	review := &ReviewState{UserID: userID, PullID: pullID, CommitSHA: commitSHA}
+	has, err := db.GetEngine(ctx).Get(review)
+	return review, has, err
+}
+
+// UpdateReviewState updates the given review inside the database, regardless of whether it existed before or not
+// The given map of files with their viewed state will be merged with the previous review, if present
+func UpdateReviewState(ctx context.Context, userID, pullID int64, commitSHA string, updatedFiles map[string]ViewedState) error {
+	log.Trace("Updating review for user %d, repo %d, commit %s with the updated files %v.", userID, pullID, commitSHA, updatedFiles)
+
+	review, exists, err := GetReviewState(ctx, userID, pullID, commitSHA)
+	if err != nil {
+		return err
+	}
+
+	if exists {
+		review.UpdatedFiles = mergeFiles(review.UpdatedFiles, updatedFiles)
+	} else if previousReview, err := getNewestReviewStateApartFrom(ctx, userID, pullID, commitSHA); err != nil {
+		return err
+
+		// Overwrite the viewed files of the previous review if present
+	} else if previousReview != nil {
+		review.UpdatedFiles = mergeFiles(previousReview.UpdatedFiles, updatedFiles)
+	} else {
+		review.UpdatedFiles = updatedFiles
+	}
+
+	// Insert or Update review
+	engine := db.GetEngine(ctx)
+	if !exists {
+		log.Trace("Inserting new review for user %d, repo %d, commit %s with the updated files %v.", userID, pullID, commitSHA, review.UpdatedFiles)
+		_, err := engine.Insert(review)
+		return err
+	}
+	log.Trace("Updating already existing review with ID %d (user %d, repo %d, commit %s) with the updated files %v.", review.ID, userID, pullID, commitSHA, review.UpdatedFiles)
+	_, err = engine.ID(review.ID).Update(&ReviewState{UpdatedFiles: review.UpdatedFiles})
+	return err
+}
+
+// mergeFiles merges the given maps of files with their viewing state into one map.
+// Values from oldFiles will be overridden with values from newFiles
+func mergeFiles(oldFiles, newFiles map[string]ViewedState) map[string]ViewedState {
+	if oldFiles == nil {
+		return newFiles
+	} else if newFiles == nil {
+		return oldFiles
+	}
+
+	for file, viewed := range newFiles {
+		oldFiles[file] = viewed
+	}
+	return oldFiles
+}
+
+// GetNewestReviewState gets the newest review of the current user in the current PR.
+// The returned PR Review will be nil if the user has not yet reviewed this PR.
+func GetNewestReviewState(ctx context.Context, userID, pullID int64) (*ReviewState, error) {
+	var review ReviewState
+	has, err := db.GetEngine(ctx).Where("user_id = ?", userID).And("pull_id = ?", pullID).OrderBy("updated_unix DESC").Get(&review)
+	if err != nil || !has {
+		return nil, err
+	}
+	return &review, err
+}
+
+// getNewestReviewStateApartFrom is like GetNewestReview, except that the second newest review will be returned if the newest review points at the given commit.
+// The returned PR Review will be nil if the user has not yet reviewed this PR.
+func getNewestReviewStateApartFrom(ctx context.Context, userID, pullID int64, commitSHA string) (*ReviewState, error) {
+	var reviews []ReviewState
+	err := db.GetEngine(ctx).Where("user_id = ?", userID).And("pull_id = ?", pullID).OrderBy("updated_unix DESC").Limit(2).Find(&reviews)
+	// It would also be possible to use ".And("commit_sha != ?", commitSHA)" instead of the error handling below
+	// However, benchmarks show drastically improved performance by not doing that
+
+	// Error cases in which no review should be returned
+	if err != nil || len(reviews) == 0 || (len(reviews) == 1 && reviews[0].CommitSHA == commitSHA) {
+		return nil, err
+
+		// The first review points at the commit to exclude, hence skip to the second review
+	} else if len(reviews) >= 2 && reviews[0].CommitSHA == commitSHA {
+		return &reviews[1], nil
+	}
+
+	// As we have no error cases left, the result must be the first element in the list
+	return &reviews[0], nil
+}
diff --git a/modules/git/repo_compare.go b/modules/git/repo_compare.go
index f6b4f77645..4b0cc8536b 100644
--- a/modules/git/repo_compare.go
+++ b/modules/git/repo_compare.go
@@ -286,6 +286,15 @@ func (repo *Repository) GetPatch(base, head string, w io.Writer) error {
 	return err
 }
 
+// GetFilesChangedBetween returns a list of all files that have been changed between the given commits
+func (repo *Repository) GetFilesChangedBetween(base, head string) ([]string, error) {
+	stdout, _, err := NewCommand(repo.Ctx, "diff", "--name-only", base+".."+head).RunStdString(&RunOpts{Dir: repo.Path})
+	if err != nil {
+		return nil, err
+	}
+	return strings.Split(stdout, "\n"), err
+}
+
 // GetDiffFromMergeBase generates and return patch data from merge base to head
 func (repo *Repository) GetDiffFromMergeBase(base, head string, w io.Writer) error {
 	stderr := new(bytes.Buffer)
diff --git a/options/locale/locale_en-US.ini b/options/locale/locale_en-US.ini
index 271fa62953..8c7269703f 100644
--- a/options/locale/locale_en-US.ini
+++ b/options/locale/locale_en-US.ini
@@ -1493,6 +1493,9 @@ pulls.allow_edits_from_maintainers = Allow edits from maintainers
 pulls.allow_edits_from_maintainers_desc = Users with write access to the base branch can also push to this branch
 pulls.allow_edits_from_maintainers_err = Updating failed
 pulls.compare_changes_desc = Select the branch to merge into and the branch to pull from.
+pulls.has_viewed_file = Viewed
+pulls.has_changed_since_last_review = Changed since your last review
+pulls.viewed_files_label = %[1]d / %[2]d files viewed
 pulls.compare_base = merge into
 pulls.compare_compare = pull from
 pulls.switch_comparison_type = Switch comparison type
diff --git a/routers/web/repo/pull.go b/routers/web/repo/pull.go
index 7cedeec10e..a0b3567738 100644
--- a/routers/web/repo/pull.go
+++ b/routers/web/repo/pull.go
@@ -685,22 +685,35 @@ func ViewPullFiles(ctx *context.Context) {
 	if fileOnly && (len(files) == 2 || len(files) == 1) {
 		maxLines, maxFiles = -1, -1
 	}
-
-	diff, err := gitdiff.GetDiff(gitRepo,
-		&gitdiff.DiffOptions{
-			BeforeCommitID:     startCommitID,
-			AfterCommitID:      endCommitID,
-			SkipTo:             ctx.FormString("skip-to"),
-			MaxLines:           maxLines,
-			MaxLineCharacters:  setting.Git.MaxGitDiffLineCharacters,
-			MaxFiles:           maxFiles,
-			WhitespaceBehavior: gitdiff.GetWhitespaceFlag(ctx.Data["WhitespaceBehavior"].(string)),
-		}, ctx.FormStrings("files")...)
+	diffOptions := &gitdiff.DiffOptions{
+		BeforeCommitID:     startCommitID,
+		AfterCommitID:      endCommitID,
+		SkipTo:             ctx.FormString("skip-to"),
+		MaxLines:           maxLines,
+		MaxLineCharacters:  setting.Git.MaxGitDiffLineCharacters,
+		MaxFiles:           maxFiles,
+		WhitespaceBehavior: gitdiff.GetWhitespaceFlag(ctx.Data["WhitespaceBehavior"].(string)),
+	}
+
+	var methodWithError string
+	var diff *gitdiff.Diff
+	if !ctx.IsSigned {
+		diff, err = gitdiff.GetDiff(gitRepo, diffOptions, files...)
+		methodWithError = "GetDiff"
+	} else {
+		diff, err = gitdiff.SyncAndGetUserSpecificDiff(ctx, ctx.Doer.ID, pull, gitRepo, diffOptions, files...)
+		methodWithError = "SyncAndGetUserSpecificDiff"
+	}
 	if err != nil {
-		ctx.ServerError("GetDiffRangeWithWhitespaceBehavior", err)
+		ctx.ServerError(methodWithError, err)
 		return
 	}
 
+	ctx.PageData["prReview"] = map[string]interface{}{
+		"numberOfFiles":       diff.NumFiles,
+		"numberOfViewedFiles": diff.NumViewedFiles,
+	}
+
 	if err = diff.LoadComments(ctx, issue, ctx.Doer); err != nil {
 		ctx.ServerError("LoadComments", err)
 		return
diff --git a/routers/web/repo/pull_review.go b/routers/web/repo/pull_review.go
index 939b0037a0..98272ed48d 100644
--- a/routers/web/repo/pull_review.go
+++ b/routers/web/repo/pull_review.go
@@ -9,8 +9,10 @@ import (
 	"net/http"
 
 	"code.gitea.io/gitea/models"
+	pull_model "code.gitea.io/gitea/models/pull"
 	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/json"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/web"
@@ -242,3 +244,47 @@ func DismissReview(ctx *context.Context) {
 
 	ctx.Redirect(fmt.Sprintf("%s/pulls/%d#%s", ctx.Repo.RepoLink, comm.Issue.Index, comm.HashTag()))
 }
+
+// viewedFilesUpdate Struct to parse the body of a request to update the reviewed files of a PR
+// If you want to implement an API to update the review, simply move this struct into modules.
+type viewedFilesUpdate struct {
+	Files         map[string]bool `json:"files"`
+	HeadCommitSHA string          `json:"headCommitSHA"`
+}
+
+func UpdateViewedFiles(ctx *context.Context) {
+	// Find corresponding PR
+	issue := checkPullInfo(ctx)
+	if ctx.Written() {
+		return
+	}
+	pull := issue.PullRequest
+
+	var data *viewedFilesUpdate
+	err := json.NewDecoder(ctx.Req.Body).Decode(&data)
+	if err != nil {
+		log.Warn("Attempted to update a review but could not parse request body: %v", err)
+		ctx.Resp.WriteHeader(http.StatusBadRequest)
+		return
+	}
+
+	// Expect the review to have been now if no head commit was supplied
+	if data.HeadCommitSHA == "" {
+		data.HeadCommitSHA = pull.HeadCommitID
+	}
+
+	updatedFiles := make(map[string]pull_model.ViewedState, len(data.Files))
+	for file, viewed := range data.Files {
+
+		// Only unviewed and viewed are possible, has-changed can not be set from the outside
+		state := pull_model.Unviewed
+		if viewed {
+			state = pull_model.Viewed
+		}
+		updatedFiles[file] = state
+	}
+
+	if err := pull_model.UpdateReviewState(ctx, ctx.Doer.ID, pull.ID, data.HeadCommitSHA, updatedFiles); err != nil {
+		ctx.ServerError("UpdateReview", err)
+	}
+}
diff --git a/routers/web/web.go b/routers/web/web.go
index dcaad3d2bd..38754025ee 100644
--- a/routers/web/web.go
+++ b/routers/web/web.go
@@ -849,6 +849,7 @@ func RegisterRoutes(m *web.Route) {
 				m.Post("/deadline", bindIgnErr(structs.EditDeadlineOption{}), repo.UpdateIssueDeadline)
 				m.Post("/watch", repo.IssueWatch)
 				m.Post("/ref", repo.UpdateIssueRef)
+				m.Post("/viewed-files", repo.UpdateViewedFiles)
 				m.Group("/dependency", func() {
 					m.Post("/add", repo.AddDependency)
 					m.Post("/delete", repo.RemoveDependency)
diff --git a/services/gitdiff/gitdiff.go b/services/gitdiff/gitdiff.go
index 1df16e5016..7fe056a481 100644
--- a/services/gitdiff/gitdiff.go
+++ b/services/gitdiff/gitdiff.go
@@ -22,6 +22,7 @@ import (
 
 	"code.gitea.io/gitea/models"
 	"code.gitea.io/gitea/models/db"
+	pull_model "code.gitea.io/gitea/models/pull"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/analyze"
 	"code.gitea.io/gitea/modules/charset"
@@ -602,25 +603,27 @@ func (diffSection *DiffSection) GetComputedInlineDiffFor(diffLine *DiffLine) Dif
 
 // DiffFile represents a file diff.
 type DiffFile struct {
-	Name                    string
-	OldName                 string
-	Index                   int
-	Addition, Deletion      int
-	Type                    DiffFileType
-	IsCreated               bool
-	IsDeleted               bool
-	IsBin                   bool
-	IsLFSFile               bool
-	IsRenamed               bool
-	IsAmbiguous             bool
-	IsSubmodule             bool
-	Sections                []*DiffSection
-	IsIncomplete            bool
-	IsIncompleteLineTooLong bool
-	IsProtected             bool
-	IsGenerated             bool
-	IsVendored              bool
-	Language                string
+	Name                      string
+	OldName                   string
+	Index                     int
+	Addition, Deletion        int
+	Type                      DiffFileType
+	IsCreated                 bool
+	IsDeleted                 bool
+	IsBin                     bool
+	IsLFSFile                 bool
+	IsRenamed                 bool
+	IsAmbiguous               bool
+	IsSubmodule               bool
+	Sections                  []*DiffSection
+	IsIncomplete              bool
+	IsIncompleteLineTooLong   bool
+	IsProtected               bool
+	IsGenerated               bool
+	IsVendored                bool
+	IsViewed                  bool // User specific
+	HasChangedSinceLastReview bool // User specific
+	Language                  string
 }
 
 // GetType returns type of diff file.
@@ -663,6 +666,18 @@ func (diffFile *DiffFile) GetTailSection(gitRepo *git.Repository, leftCommitID,
 	return tailSection
 }
 
+// GetDiffFileName returns the name of the diff file, or its old name in case it was deleted
+func (diffFile *DiffFile) GetDiffFileName() string {
+	if diffFile.Name == "" {
+		return diffFile.OldName
+	}
+	return diffFile.Name
+}
+
+func (diffFile *DiffFile) ShouldBeHidden() bool {
+	return diffFile.IsGenerated || diffFile.IsViewed
+}
+
 func getCommitFileLineCount(commit *git.Commit, filePath string) int {
 	blob, err := commit.GetBlobByPath(filePath)
 	if err != nil {
@@ -677,10 +692,12 @@ func getCommitFileLineCount(commit *git.Commit, filePath string) int {
 
 // Diff represents a difference between two git trees.
 type Diff struct {
-	Start, End                             string
-	NumFiles, TotalAddition, TotalDeletion int
-	Files                                  []*DiffFile
-	IsIncomplete                           bool
+	Start, End                   string
+	NumFiles                     int
+	TotalAddition, TotalDeletion int
+	Files                        []*DiffFile
+	IsIncomplete                 bool
+	NumViewedFiles               int // user-specific
 }
 
 // LoadComments loads comments into each line
@@ -1497,6 +1514,70 @@ func GetDiff(gitRepo *git.Repository, opts *DiffOptions, files ...string) (*Diff
 	return diff, nil
 }
 
+// SyncAndGetUserSpecificDiff is like GetDiff, except that user specific data such as which files the given user has already viewed on the given PR will also be set
+// Additionally, the database asynchronously is updated if files have changed since the last review
+func SyncAndGetUserSpecificDiff(ctx context.Context, userID int64, pull *models.PullRequest, gitRepo *git.Repository, opts *DiffOptions, files ...string) (*Diff, error) {
+	diff, err := GetDiff(gitRepo, opts, files...)
+	if err != nil {
+		return nil, err
+	}
+	review, err := pull_model.GetNewestReviewState(ctx, userID, pull.ID)
+	if err != nil || review == nil || review.UpdatedFiles == nil {
+		return diff, err
+	}
+
+	latestCommit := opts.AfterCommitID
+	if latestCommit == "" {
+		latestCommit = pull.HeadBranch // opts.AfterCommitID is preferred because it handles PRs from forks correctly and the branch name doesn't
+	}
+
+	changedFiles, err := gitRepo.GetFilesChangedBetween(review.CommitSHA, latestCommit)
+	if err != nil {
+		return diff, err
+	}
+
+	filesChangedSinceLastDiff := make(map[string]pull_model.ViewedState)
+outer:
+	for _, diffFile := range diff.Files {
+		fileViewedState := review.UpdatedFiles[diffFile.GetDiffFileName()]
+
+		// Check whether it was previously detected that the file has changed since the last review
+		if fileViewedState == pull_model.HasChanged {
+			diffFile.HasChangedSinceLastReview = true
+			continue
+		}
+
+		filename := diffFile.GetDiffFileName()
+
+		// Check explicitly whether the file has changed since the last review
+		for _, changedFile := range changedFiles {
+			diffFile.HasChangedSinceLastReview = filename == changedFile
+			if diffFile.HasChangedSinceLastReview {
+				filesChangedSinceLastDiff[filename] = pull_model.HasChanged
+				continue outer // We don't want to check if the file is viewed here as that would fold the file, which is in this case unwanted
+			}
+		}
+		// Check whether the file has already been viewed
+		if fileViewedState == pull_model.Viewed {
+			diffFile.IsViewed = true
+			diff.NumViewedFiles++
+		}
+	}
+
+	// Explicitly store files that have changed in the database, if any is present at all.
+	// This has the benefit that the "Has Changed" attribute will be present as long as the user does not explicitly mark this file as viewed, so it will even survive a page reload after marking another file as viewed.
+	// On the other hand, this means that even if a commit reverting an unseen change is committed, the file will still be seen as changed.
+	if len(filesChangedSinceLastDiff) > 0 {
+		err := pull_model.UpdateReviewState(ctx, review.UserID, review.PullID, review.CommitSHA, filesChangedSinceLastDiff)
+		if err != nil {
+			log.Warn("Could not update review for user %d, pull %d, commit %s and the changed files %v: %v", review.UserID, review.PullID, review.CommitSHA, filesChangedSinceLastDiff, err)
+			return nil, err
+		}
+	}
+
+	return diff, err
+}
+
 // CommentAsDiff returns c.Patch as *Diff
 func CommentAsDiff(c *models.Comment) (*Diff, error) {
 	diff, err := ParsePatch(setting.Git.MaxGitDiffLines,
diff --git a/templates/repo/diff/box.tmpl b/templates/repo/diff/box.tmpl
index 6f67f36422..d6ad169821 100644
--- a/templates/repo/diff/box.tmpl
+++ b/templates/repo/diff/box.tmpl
@@ -18,6 +18,12 @@
 				{{svg "octicon-diff" 16 "mr-2"}}{{.i18n.Tr "repo.diff.stats_desc" .Diff.NumFiles .Diff.TotalAddition .Diff.TotalDeletion | Str2html}}
 			</div>
 			<div class="diff-detail-actions df ac">
+				{{if and .PageIsPullFiles $.SignedUserID (not .IsArchived)}}
+					<meter id="viewed-files-summary" value="{{.Diff.NumViewedFiles}}" max="{{.Diff.NumFiles}}"></meter>
+					<label for="viewed-files-summary" id="viewed-files-summary-label" data-text-changed-template="{{.i18n.Tr "repo.pulls.viewed_files_label"}}">
+						{{.i18n.Tr "repo.pulls.viewed_files_label" .Diff.NumViewedFiles .Diff.NumFiles}}
+					</label>
+				{{end}}
 				{{template "repo/diff/whitespace_dropdown" .}}
 				{{template "repo/diff/options_dropdown" .}}
 				{{if and .PageIsPullFiles $.SignedUserID (not .IsArchived)}}
@@ -58,11 +64,11 @@
 				{{$isCsv := (call $.IsCsvFile $file)}}
 				{{$showFileViewToggle := or $isImage (and (not $file.IsIncomplete) $isCsv)}}
 				{{$nameHash := Sha1 $file.Name}}
-				<div class="diff-file-box diff-box file-content {{TabSizeClass $.Editorconfig $file.Name}} mt-3" id="diff-{{$nameHash}}" data-old-filename="{{$file.OldName}}" data-new-filename="{{$file.Name}}" {{if $file.IsGenerated}}data-folded="true"{{end}}>
+				<div class="diff-file-box diff-box file-content {{TabSizeClass $.Editorconfig $file.Name}} mt-3" id="diff-{{$nameHash}}" data-old-filename="{{$file.OldName}}" data-new-filename="{{$file.Name}}" {{if $file.ShouldBeHidden}}data-folded="true"{{end}}>
 					<h4 class="diff-file-header sticky-2nd-row ui top attached normal header df ac sb">
 						<div class="df ac">
 							<a role="button" class="fold-file muted mr-2">
-								{{if $file.IsGenerated}}
+								{{if $file.ShouldBeHidden}}
 									{{svg "octicon-chevron-right" 18}}
 								{{else}}
 									{{svg "octicon-chevron-down" 18}}
@@ -106,9 +112,18 @@
 									<a class="ui basic tiny button" rel="nofollow" href="{{$.SourcePath}}/{{PathEscapeSegments .Name}}">{{$.i18n.Tr "repo.diff.view_file"}}</a>
 								{{end}}
 							{{end}}
+							{{if and $.IsSigned $.PageIsPullFiles (not $.IsArchived)}}
+								{{if $file.HasChangedSinceLastReview}}
+									<span class="changed-since-last-review unselectable">{{$.i18n.Tr "repo.pulls.has_changed_since_last_review"}}</span>
+								{{end}}
+								<div data-link="{{$.Issue.Link}}/viewed-files" data-headcommit="{{$.PullHeadCommitID}}" class="viewed-file-form unselectable{{if $file.IsViewed}} viewed-file-checked-form{{end}}">
+									<input type="checkbox" name="{{$file.GetDiffFileName}}" id="viewed-file-checkbox-{{$i}}" autocomplete="off" {{if $file.IsViewed}}checked{{end}}></input>
+									<label for="viewed-file-checkbox-{{$i}}">{{$.i18n.Tr "repo.pulls.has_viewed_file"}}</label>
+								</div>
+							{{end}}
 						</div>
 					</h4>
-					<div class="diff-file-body ui attached unstackable table segment">
+					<div class="diff-file-body ui attached unstackable table segment" {{if $file.IsViewed}}data-folded="true"{{end}}>
 						<div id="diff-source-{{$i}}" class="file-body file-code unicode-escaped code-diff{{if $.IsSplitStyle}} code-diff-split{{else}} code-diff-unified{{end}}{{if $showFileViewToggle}} hide{{end}}">
 							{{if or $file.IsIncomplete $file.IsBin}}
 								<div class="diff-file-body binary" style="padding: 5px 10px;">
diff --git a/web_src/js/features/file-fold.js b/web_src/js/features/file-fold.js
new file mode 100644
index 0000000000..5e714a1de8
--- /dev/null
+++ b/web_src/js/features/file-fold.js
@@ -0,0 +1,18 @@
+import {svg} from '../svg.js';
+
+
+// Hides the file if newFold is true, and shows it otherwise. The actual hiding is performed using CSS.
+//
+// The fold arrow is the icon displayed on the upper left of the file box, especially intended for components having the 'fold-file' class.
+// The file content box is the box that should be hidden or shown, especially intended for components having the 'file-content' class.
+//
+export function setFileFolding(fileContentBox, foldArrow, newFold) {
+  foldArrow.innerHTML = svg(`octicon-chevron-${newFold ? 'right' : 'down'}`, 18);
+  fileContentBox.setAttribute('data-folded', newFold);
+}
+
+// Like `setFileFolding`, except that it automatically inverts the current file folding state.
+export function invertFileFolding(fileContentBox, foldArrow) {
+  setFileFolding(fileContentBox, foldArrow, fileContentBox.getAttribute('data-folded') !== 'true');
+}
+
diff --git a/web_src/js/features/pull-view-file.js b/web_src/js/features/pull-view-file.js
new file mode 100644
index 0000000000..57e9d5e047
--- /dev/null
+++ b/web_src/js/features/pull-view-file.js
@@ -0,0 +1,71 @@
+import {setFileFolding} from './file-fold.js';
+
+const {csrfToken, pageData} = window.config;
+const prReview = pageData.prReview || {};
+const viewedStyleClass = 'viewed-file-checked-form';
+const viewedCheckboxSelector = '.viewed-file-form'; // Selector under which all "Viewed" checkbox forms can be found
+
+
+// Refreshes the summary of viewed files if present
+// The data used will be window.config.pageData.prReview.numberOf{Viewed}Files
+function refreshViewedFilesSummary() {
+  const viewedFilesMeter = document.getElementById('viewed-files-summary');
+  viewedFilesMeter?.setAttribute('value', prReview.numberOfViewedFiles);
+  const summaryLabel = document.getElementById('viewed-files-summary-label');
+  if (summaryLabel) summaryLabel.innerHTML = summaryLabel.getAttribute('data-text-changed-template')
+    .replace('%[1]d', prReview.numberOfViewedFiles)
+    .replace('%[2]d', prReview.numberOfFiles);
+}
+
+// Explicitly recounts how many files the user has currently reviewed by counting the number of checked "viewed" checkboxes
+// Additionally, the viewed files summary will be updated if it exists
+export function countAndUpdateViewedFiles() {
+  // The number of files is constant, but the number of viewed files can change because files can be loaded dynamically
+  prReview.numberOfViewedFiles = document.querySelectorAll(`${viewedCheckboxSelector} > input[type=checkbox][checked]`).length;
+  refreshViewedFilesSummary();
+}
+
+// Initializes a listener for all children of the given html element
+// (for example 'document' in the most basic case)
+// to watch for changes of viewed-file checkboxes
+export function initViewedCheckboxListenerFor() {
+  for (const form of document.querySelectorAll(`${viewedCheckboxSelector}:not([data-has-viewed-checkbox-listener="true"])`)) {
+    // To prevent double addition of listeners
+    form.setAttribute('data-has-viewed-checkbox-listener', true);
+
+    // The checkbox consists of a div containing the real checkbox with its label and the CSRF token,
+    // hence the actual checkbox first has to be found
+    const checkbox = form.querySelector('input[type=checkbox]');
+    checkbox.addEventListener('change', function() {
+      // Mark the file as viewed visually - will especially change the background
+      if (this.checked) {
+        form.classList.add(viewedStyleClass);
+        prReview.numberOfViewedFiles++;
+      } else {
+        form.classList.remove(viewedStyleClass);
+        prReview.numberOfViewedFiles--;
+      }
+
+      // Update viewed-files summary and remove "has changed" label if present
+      refreshViewedFilesSummary();
+      const hasChangedLabel = form.parentNode.querySelector('.changed-since-last-review');
+      hasChangedLabel?.parentNode.removeChild(hasChangedLabel);
+
+      // Unfortunately, actual forms cause too many problems, hence another approach is needed
+      const files = {};
+      files[checkbox.getAttribute('name')] = this.checked;
+      const data = {files};
+      const headCommitSHA = form.getAttribute('data-headcommit');
+      if (headCommitSHA) data.headCommitSHA = headCommitSHA;
+      fetch(form.getAttribute('data-link'), {
+        method: 'POST',
+        headers: {'X-Csrf-Token': csrfToken},
+        body: JSON.stringify(data),
+      });
+
+      // Fold the file accordingly
+      const parentBox = form.closest('.diff-file-header');
+      setFileFolding(parentBox.closest('.file-content'), parentBox.querySelector('.fold-file'), this.checked);
+    });
+  }
+}
diff --git a/web_src/js/features/repo-code.js b/web_src/js/features/repo-code.js
index d7b4baac83..8562ba0072 100644
--- a/web_src/js/features/repo-code.js
+++ b/web_src/js/features/repo-code.js
@@ -1,5 +1,6 @@
 import $ from 'jquery';
 import {svg} from '../svg.js';
+import {invertFileFolding} from './file-fold.js';
 
 function changeHash(hash) {
   if (window.history.pushState) {
@@ -148,10 +149,7 @@ export function initRepoCodeView() {
     }).trigger('hashchange');
   }
   $(document).on('click', '.fold-file', ({currentTarget}) => {
-    const box = currentTarget.closest('.file-content');
-    const folded = box.getAttribute('data-folded') !== 'true';
-    currentTarget.innerHTML = svg(`octicon-chevron-${folded ? 'right' : 'down'}`, 18);
-    box.setAttribute('data-folded', String(folded));
+    invertFileFolding(currentTarget.closest('.file-content'), currentTarget);
   });
   $(document).on('click', '.blob-excerpt', async ({currentTarget}) => {
     const url = currentTarget.getAttribute('data-url');
diff --git a/web_src/js/features/repo-diff.js b/web_src/js/features/repo-diff.js
index 013b8d3fa0..8403a2fd42 100644
--- a/web_src/js/features/repo-diff.js
+++ b/web_src/js/features/repo-diff.js
@@ -2,6 +2,7 @@ import $ from 'jquery';
 import {initCompReactionSelector} from './comp/ReactionSelector.js';
 import {initRepoIssueContentHistory} from './repo-issue-content.js';
 import {validateTextareaNonEmpty} from './comp/EasyMDE.js';
+import {initViewedCheckboxListenerFor, countAndUpdateViewedFiles} from './pull-view-file.js';
 
 const {csrfToken} = window.config;
 
@@ -104,6 +105,13 @@ export function initRepoDiffConversationNav() {
   });
 }
 
+// Will be called when the show more (files) button has been pressed
+function onShowMoreFiles() {
+  initRepoIssueContentHistory();
+  initViewedCheckboxListenerFor();
+  countAndUpdateViewedFiles();
+}
+
 export function initRepoDiffShowMore() {
   $('#diff-files, #diff-file-boxes').on('click', '#diff-show-more-files, #diff-show-more-files-stats', (e) => {
     e.preventDefault();
@@ -125,7 +133,7 @@ export function initRepoDiffShowMore() {
       $('#diff-too-many-files-stats').remove();
       $('#diff-files').append($(resp).find('#diff-files li'));
       $('#diff-incomplete').replaceWith($(resp).find('#diff-file-boxes').children());
-      initRepoIssueContentHistory();
+      onShowMoreFiles();
     }).fail(() => {
       $('#diff-show-more-files, #diff-show-more-files-stats').removeClass('disabled');
     });
@@ -151,7 +159,7 @@ export function initRepoDiffShowMore() {
       }
 
       $target.parent().replaceWith($(resp).find('#diff-file-boxes .diff-file-body .file-body').children());
-      initRepoIssueContentHistory();
+      onShowMoreFiles();
     }).fail(() => {
       $target.removeClass('disabled');
     });
diff --git a/web_src/js/index.js b/web_src/js/index.js
index 5b95a0d8ef..4343c2d965 100644
--- a/web_src/js/index.js
+++ b/web_src/js/index.js
@@ -70,6 +70,7 @@ import {
   initRepoSettingsCollaboration,
   initRepoSettingSearchTeamBox,
 } from './features/repo-settings.js';
+import {initViewedCheckboxListenerFor} from './features/pull-view-file.js';
 import {initOrgTeamSearchRepoBox, initOrgTeamSettings} from './features/org-team.js';
 import {initUserAuthWebAuthn, initUserAuthWebAuthnRegister} from './features/user-auth-webauthn.js';
 import {initRepoRelease, initRepoReleaseEditor} from './features/repo-release.js';
@@ -178,6 +179,6 @@ $(document).ready(() => {
   initUserAuthWebAuthn();
   initUserAuthWebAuthnRegister();
   initUserSettings();
-
+  initViewedCheckboxListenerFor();
   checkAppUrl();
 });
diff --git a/web_src/less/_review.less b/web_src/less/_review.less
index 8bc41b9736..1bcb3663c2 100644
--- a/web_src/less/_review.less
+++ b/web_src/less/_review.less
@@ -262,3 +262,21 @@ a.blob-excerpt:hover {
     scroll-margin-top: 130px;
   }
 }
+
+.changed-since-last-review {
+  margin: 0 5px;
+  padding: 0 3px;
+  border: 2px var(--color-primary-light-3) solid;
+  background-color: var(--color-primary-alpha-30);
+  border-radius: 7px;
+}
+
+.viewed-file-form {
+  margin: 0 3px;
+  padding: 0 3px;
+  border-radius: 3px;
+}
+
+.viewed-file-checked-form {
+  background-color: var(--color-primary-light-4);
+}