Was able to solve it myself.
After some digging in the manuals, it seems that AGS is actually calculating based on the height of the GUI bg and the inventory thumbnail height, if the item is added to the second line or not.
The height of my item thumbnails was 21, the height of the GUIControl Object 41. 41 / 31 < 2 so, only one Row was filled.
A good example to make simple things extremely complicated, but at least I know the trick now ;-)
After some digging in the manuals, it seems that AGS is actually calculating based on the height of the GUI bg and the inventory thumbnail height, if the item is added to the second line or not.
QuoteRowCount property (inv window)
readonly int InvWindow.RowCount;
Gets the number of rows that can be displayed within the inventory window. This is calculated by dividing the height of the window by the individual ItemHeight.
Example:
Display("The inventory window can show %d items at a time", invMain.ItemsPerRow * invMain.RowCount);
displays how many items can be visible in the invMain window at once.
See Also: InvWindow.ItemHeight, InvWindow.ItemsPerRow
The height of my item thumbnails was 21, the height of the GUIControl Object 41. 41 / 31 < 2 so, only one Row was filled.
A good example to make simple things extremely complicated, but at least I know the trick now ;-)