If hide columns and overlap partial merged cell, cells' alignment will be chaos if we don't unhide all columns.
(horizontal merged cell only)

reproducing steps:

1. merge cells in a row (e.g. A5:C5)
2. select two or more columns and the first column has overlap a merged cell (e.g. A:B)
3. hide these columns
4. resize column (e.g. B) to unhide one column by mouse (not by context menu)
   • the cells' size will be chaos until unhide all columns