Correct. Portals cannot be scrolled horizontally.
Given the size of monitor screens, the idea of a portal so wide that it can't show all fields in the portal with room to spare would have me re-evaluating my design as that seems like way too many fields in one row to work with easily.
But maybe this is for the much smaller screen of an iPhone?
What you could do is set up a slide control where each panel of the control has a portal with a different sub set of the total set of fields. you can then move from panel to panel.
It's pretty common if you use Salesforce or Dynamics, they all work that way. I have Salesforce open right now for our partner company who has 5000+ employees. 9 columns are visible, then I keep scrolling to the right and can see 15 in total. It's much cleaner since each column can actually be wide to show the data, vs crammed in super small to make it fit.
Typical monitor displays should easily be able to show all 15 columns without any need for scrolling.
Well they don't unfortunately. There's a big difference between machines at 1366x768 and 1920x1080. You're basically forced to design layouts for the smallest resolution screen, then have them waste space and stretch on larger ones, or you design them for the larger ones as they are now, and then users have to scroll the whole window up and down side to side if they are on a lower resolution system where the layouts don't fit.
I'm looking a pretty small screen here on my lap top and thinking that as a user, I really wouldn't want to have to scroll more than the width of this screen to see data as rows of data in tabular format that wide would be a real pain in the neck to work with. But hey, that's just me expressing an opinion.
It occurs to me that the horizontal scroll that you want could very well be possible. Just replace the portal with a list view. You can change your layout context from the parent record to the portals, but still put fields from the parent record in the header. The challenge would be to replace your navigation system that previously just had to move from parent record to parent record with scripted buttons that pull up sorted found sets of child records for the next, prev, first, etc parents and any user performed finds would need to be constrained to finds for a given parent record in some cases.
But with list view, you can scroll as far to the right as you want.