fix(projects): move only the dragged card on project boards (#40182)

The board reported every card of the target column on each drag, and the
server wrote all of them into that column. A stale board in another
browser therefore pulled cards moved by someone else back to their old
column.

The client now sends only the dragged issue and its predecessor, and the
server computes the position in one transaction:

- Only the dragged issue changes column, the others are just renumbered.
- A predecessor that is no longer in the column puts the card at the
end.

Added an e2e test.

Fixes https://github.com/go-gitea/gitea/issues/21243
Fixes https://github.com/go-gitea/gitea/issues/32070
Fixes https://github.com/go-gitea/gitea/issues/13498

---
Assisted-by: Codet:claude

---------

Co-authored-by: silverwind <me@silverwind.io>

services/projects/issue.go

@@ -99,10 +99,35 @@ func RemoveIssueFromColumn(ctx context.Context, doer *user_model.User, issue *is
 	return nil
 }
 
-// MoveIssuesOnProjectColumn moves or keeps issues in a column and sorts them inside that column
-func MoveIssuesOnProjectColumn(ctx context.Context, doer *user_model.User, column *project_model.Column, sortedIssueIDs map[int64]int64) error {
+// MoveIssueAfter places the issue after prevIssueID, first for 0, last when it left the column.
+func MoveIssueAfter(ctx context.Context, doer *user_model.User, column *project_model.Column, issue *issues_model.Issue, prevIssueID int64) error {
 	moves, err := db.WithTx2(ctx, func(ctx context.Context) ([]columnMove, error) {
-		return moveIssuesOnProjectColumn(ctx, doer, column, sortedIssueIDs)
+		if err := project_model.LockProject(ctx, column.ProjectID); err != nil {
+			return nil, err
+		}
+		others, err := project_model.GetColumnIssueSortings(ctx, column, issue.ID)
+		if err != nil {
+			return nil, err
+		}
+		pos := util.Iif(prevIssueID == 0, 0, len(others))
+		if i := slices.IndexFunc(others, func(row project_model.ColumnIssueSorting) bool { return row.IssueID == prevIssueID }); i >= 0 {
+			pos = i + 1
+		}
+
+		moves, err := moveIssuesOnProjectColumn(ctx, doer, column, map[int64]int64{int64(pos): issue.ID})
+		if err != nil {
+			return nil, err
+		}
+		for i, other := range others {
+			newSorting := int64(i + util.Iif(i >= pos, 1, 0))
+			if other.Sorting == newSorting {
+				continue
+			}
+			if err := project_model.SetIssueSorting(ctx, column, other.IssueID, newSorting); err != nil {
+				return nil, err
+			}
+		}
+		return moves, nil
 	})
 	if err != nil {
 		return err

services/projects/issue_test.go

@@ -156,7 +156,7 @@ func Test_Projects(t *testing.T) {
 		// move the issue into a non-default column of the first project only
 		targetColumn := &project_model.Column{Title: "target", ProjectID: projects[0].ID}
 		assert.NoError(t, project_model.NewColumn(t.Context(), targetColumn))
-		assert.NoError(t, MoveIssuesOnProjectColumn(t.Context(), user2, targetColumn, map[int64]int64{0: issue11.ID}))
+		assert.NoError(t, MoveIssueAfter(t.Context(), user2, targetColumn, issue11, 0))
 
 		unittest.AssertExistsAndLoadBean(t, &project_model.ProjectIssue{
 			IssueID: issue11.ID, ProjectID: projects[0].ID, ProjectColumnID: targetColumn.ID,
@@ -166,6 +166,44 @@ func Test_Projects(t *testing.T) {
 		})
 	})
 
+	t.Run("Moving an issue never moves other cards", func(t *testing.T) {
+		project := &project_model.Project{Title: "incremental move", RepoID: 1, Type: project_model.TypeRepository}
+		assert.NoError(t, project_model.NewProject(t.Context(), project))
+		defer func() {
+			assert.NoError(t, project_model.DeleteProjectByID(t.Context(), project.ID))
+		}()
+		colA := &project_model.Column{Title: "A", ProjectID: project.ID}
+		colB := &project_model.Column{Title: "B", ProjectID: project.ID}
+		assert.NoError(t, project_model.NewColumn(t.Context(), colA))
+		assert.NoError(t, project_model.NewColumn(t.Context(), colB))
+		for i, id := range []int64{1, 2, 3, 5} {
+			assert.NoError(t, db.Insert(t.Context(), &project_model.ProjectIssue{IssueID: id, ProjectID: project.ID, ProjectColumnID: colA.ID, Sorting: int64(i)}))
+		}
+		issue := func(id int64) *issues_model.Issue {
+			return unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{ID: id})
+		}
+		order := func(col *project_model.Column) (ids []int64) {
+			rows, err := project_model.GetColumnIssueSortings(t.Context(), col, 0)
+			assert.NoError(t, err)
+			for i, row := range rows {
+				assert.EqualValues(t, i, row.Sorting)
+				ids = append(ids, row.IssueID)
+			}
+			return ids
+		}
+
+		assert.NoError(t, MoveIssueAfter(t.Context(), user2, colB, issue(1), 0))
+		assert.NoError(t, MoveIssueAfter(t.Context(), user2, colA, issue(2), 1))
+		assert.Equal(t, []int64{3, 5, 2}, order(colA))
+		assert.NoError(t, MoveIssueAfter(t.Context(), user2, colA, issue(5), 0))
+		assert.Equal(t, []int64{5, 3, 2}, order(colA))
+		assert.NoError(t, MoveIssueAfter(t.Context(), user2, colA, issue(5), 3))
+		assert.Equal(t, []int64{3, 5, 2}, order(colA))
+		assert.Equal(t, []int64{1}, order(colB))
+		assert.NoError(t, project_model.SetIssueSorting(t.Context(), colA, 1, 99))
+		assert.Equal(t, []int64{1}, order(colB))
+	})
+
 	t.Run("Repository projects", func(t *testing.T) {
 		repo1 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})