When expanding rows/columns in TableMatrix the underlying cells (that some cells have expanded over) still exists, although not visible. The span just overlaps nearby existing cells to resize other cell(s). This is the reason why "nothing moves down". This makes it bit of a mess to use, since we have to keep track of what cells that: a) exist but are not visible, b) spanned and visible and c) normal (unspanned and visible)..

Also the TableMatrix docs says: "When setting a span, checks are made as to whether the span would overlap an already spanning or hidden cell. This is an error and it not allowed".

This is something we have to consider when trying to do vertical spanning of cells, cos we have to make sure that span does not overlap non-expanded SpreadsheetHideRows rows.