ajaxorg/ace · error · Error

Trying to add fold to FoldRow that doesn't have a matching r

Error message

Trying to add fold to FoldRow that doesn't have a matching row

What it means

FoldLine.addFold() places a fold either at the end of the line (start.row == endRow), at the start (end.row == startRow), or as sameRow. A fold whose rows match none of these cannot belong to this FoldLine, so Ace throws to protect fold integrity.

Source

Thrown at src/edit_session/fold_line.js:75

                return -a.range.compareEnd(b.start.row, b.start.column);
            });
            if (this.range.compareEnd(fold.start.row, fold.start.column) > 0) {
                this.end.row = fold.end.row;
                this.end.column =  fold.end.column;
            } else if (this.range.compareStart(fold.end.row, fold.end.column) < 0) {
                this.start.row = fold.start.row;
                this.start.column = fold.start.column;
            }
        } else if (fold.start.row == this.end.row) {
            this.folds.push(fold);
            this.end.row = fold.end.row;
            this.end.column = fold.end.column;
        } else if (fold.end.row == this.start.row) {
            this.folds.unshift(fold);
            this.start.row = fold.start.row;
            this.start.column = fold.start.column;
        } else {
            throw new Error("Trying to add fold to FoldRow that doesn't have a matching row");
        }
        fold.foldLine = this;
    }

    /**
     * @param {number} row
     */
    containsRow(row) {
        return row >= this.start.row && row <= this.end.row;
    }

    /**
     * @param {Function} callback
     * @param {number} endRow
     * @param {number} endColumn
     */
    walk(callback, endRow, endColumn) {
        var lastEnd = 0,

View on GitHub (pinned to 2c1eddc392)

Solutions

1. Ensure fold ranges align with FoldLine boundaries (single row, or start/end matching the FoldLine's rows)
2. Use editSession.addFold / unfold APIs rather than manipulating FoldLine directly
3. Unfold and re-fold the region if line numbers shifted after edits

Example fix

// before
foldLine.addFold(new Fold(3, 0, 7, 5)); // spans beyond this fold line
// after
session.addFold('...', new AceRange(3, 0, 7, 5));

if (!(fold.end.row === foldLine.start.row || fold.start.row === foldLine.endRow || (fold.start.row === fold.end.row && fold.start.row >= foldLine.startRow && fold.end.row <= foldLine.endRow))) {
  throw new Error('fold rows do not align with FoldLine');
}

function foldFits(foldLine, fold) { return fold.start.row >= foldLine.startRow && fold.end.row <= foldLine.endRow; }

try { session.addFold('...', range); } catch (e) { if (/matching row/.test(e.message)) { session.unfold(range); session.addFold('...', range); } else throw e; }

When it happens

Trigger: Adding a fold whose row range partially overlaps a fold line (e.g. fold from row 3 to row 7 on a FoldLine covering rows 2-4); manually constructing folds across multiple FoldLines.

Common situations: Custom folding plugins computing ranges incorrectly; folding operations applied after text changed line numbers without updating fold state.

Related errors

• Can't add a fold to this FoldLine as it has no connection
• invalid fold style:

AI-assisted analysis of ajaxorg/ace@2c1eddc392 (2026-08-30). Data as JSON: /api/errors/b899145e94b9f0d3. Report an issue: GitHub.