selection box should not be highlighted when it's on a merged cell

Steps to Reproduce

1. start keikai, create a 3x2 merged cell
2. select the merged cell

Current Result

the selection box is highlighted like we select multiple cells

Expected Result

it's not highlighted in Excel in this case:

Debug Information

1.
.zsselecti-r

2. Usability impact: such highlight affects a cell text's readability for a merged cell