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To reproduce this issue:
1) Open your Column ContextMenu sample
2) Drag and drop ProductName column to make it wider to the right until Discontinued column disappears from sight.
3) Right click on any header column and uncheck column Discontinued.
4) Check again in the contextMenu column Discontinued.
5) Scroll horizontally to the right. Discontinued Column has disappeared although you have checked this column in the contextColumn.
Se attached image.
1) Open your Column ContextMenu sample
2) Drag and drop ProductName column to make it wider to the right until Discontinued column disappears from sight.
3) Right click on any header column and uncheck column Discontinued.
4) Check again in the contextMenu column Discontinued.
5) Scroll horizontally to the right. Discontinued Column has disappeared although you have checked this column in the contextColumn.
Se attached image.