LG 25UM58-P ultrawide images stretched. 2560x1080 not avail

How are you connecting the ultrawide display to the XPS, i.e. what type of cable?  Are there any docks, adapters, or dongles involved between the XPS and the display input?

Thanks for the quick reply.  Sorry for missing out on this.  I'm using a miniDisplayPort-to-HDMI adapter that goes into the LG provided HDMI cable.  I have 2 adapters - one from belkin that supports 4k and another from insignia.  The issue persists with both.  Both adapters and the same cable scaled my previous Dell XPS L321X just fine.  Both also scaled this current XPS 13 9343 also just fine until something keeps changing it back to 1920x1080 by default.  

Ok, then yes I agree that should definitely work.  I was going to suggest trying native DisplayPort just to eliminate your system having to switch to HDMI signaling, but it looks like that display only has HDMI inputs.  Did this problem just start occurring or did it work before?  The reason I ask is that the 9343 is an older system, so I thought this might be a new problem after it was previously working.  If so, since you mentioned you're up to date even on your OS, I was going to suggest that Windows 10 1809 may be the culprit, or the drivers that came with it.  Microsoft has already blocked further Win10 1809 systems on certain systems due to problems with a recent Intel Graphics driver update.  The known problem doesn't relate specifically to this issue, but on the other hand the list of issues around Windows 10 1809 and driver problems is still growing.

If this has never worked correctly and you're already current on all of your firmware and driver updates, and you've already successfully tested the adapter and cable on another system, my only "last ditch" suggestion would be to try a different HDMI cable just in case it's a borderline cable and the 9343 is somehow more affected by the issue than other systems.  I recently had an issue where I tried to reuse an existing HDMI cable when upgrading from a device that only used 1080p to a new device that used 4K HDR.  The cable seemed to work fine with that signal, except that every 10 seconds or so the entire image would blink off and then reappear very quickly afterward.  Swapping cables resulted in a stable signal.  Obviously that's not exactly the same case as yours, but HDMI cables are cheap and lots of people tend to have a few already, so it's an easy test.

Thanks for the elaborate reply.  I have actually tried another HDMI cable already, just didn't mention it.  So the only items from your list that I havent' tried is a miniDisplayPort-to-HDMI cable now.  I'll grab one today and check.

This morning, the issue went away.  As I powered up and went to display options to check things out, the 2560x1080 was magically offered as an option.  

So in other words, the issue isn't predictable and something is definitely up as I have no idea how it will scale the next time I reboot.

Will check the different cable.

Thanks for your reply anyway.  Much appreciated.

Does the monitor show with the correct designation in Device Manager?

I know you state you are using just the external display, so you turn off the laptop display?  Since you have the Intel 5500 graphics, does that mean your system has a 1920 x 1080 display?

I saw this on the Intel site. 

If you are using multiple displays in clone or duplicate mode, you can only select a resolution supported by all displays.

Thanks.  yes, monitor shows properly in the device manager.

About the resolution - yes both displays (laptop and external) support the 2560x1080

I just set up my XPS 13 9365 to connect to an LG 34 2560x1080 through a TB16 dock since I do not have an adapter cable.  When I selected to project to just the external monitor, it changed to the 2560x1080 output.

The monitor does not show up correctly in the Device Manager but I have not loaded any drivers for it on this system.

Is there anything you might want me to check?