Q: 29" Ultrawide monitor and a 2015 Macbook?

Same here. Got an MB (Early 2015) and an LG 21:9 screen. I'm using the Hootoo Shuttle hub which also supports 4K HDMI. But when I plug it in, nothing happens. Before that, on my MBP 2009 HDMI wasn't working also and I had to use a Displayport cable to get it working. I'm doubting if the Macbook can handle 21:9 resolutions

Did you read into these support articles to see if they may be of help?

• Using 4K displays and Ultra HD TVs with your Mac - Apple Support

• USB-C Digital AV Multiport Adapter

You may need to contact an Apple specialist to ask if they have info.

Good luck in this matter.

Yes I did and they both tell me that it supports up to 4k, but still it doesn't support the 21:9 ratio resolution when I try. So that doesn't really help me. I'm going to try another HDMI cable today but if that doesn't work, I'm out of options.

You could try using SwitchResX: http://www.madrau.com

It has nothing to with 4k capability, your Dell monitor is not 4k.

I tried using SwitchResX, but my Macbook doesn't show the external screen, not even in SwitchResX so I can't configure anything.

HDMI 2 vs 1.3 might be the issue: https://www.reddit.com/r/ultrawidemasterrace/comments/3vos2i/how_the_eff_do_i_ge t_2560x1080_through_hdmi/

Also make sure your USB cables are marked as High Speed HDMI.

Both links I provided were to show the supported possibilities in general,

as applicable; specific details for the 12-inch Retina MacBook are within.

The aspect ratio of the display, monitor, or television likely will not be one

supported by the computer's graphic processor. To locate an aluminum

Apple cinema display (not Thunderbolt) in the second-hand market may

be a better choice; providing one is available in good shape at fair cost.

Sorry the devices are not working together as a reliable configuration should.

Since the Dell U2913WM is a 2560x1080 display, and I'm currently using an Acer K272HUL, which is a 2560x1440 display over HDMI from my 12" MacBook, I'm thinking there are two possibilities, as Lanny mentioned - either it's an HDMI version problem, or an HDMI cable problem.

In this case, the U2913WM is a 2013 model year display, so it should support a new enough version of HDMI to allow connection with the MacBook.  The next possibility is that either there's an issue with the multiport adapter you're using (for example, I have a VTin multiport adapter that only supports 1920x1080 over the HDMI port, even though the MacBook will cheerfully go higher) or with the HDMI cable you're using.  I eventually bit the bullet and bought the Apple Digital Multiport Adapter and a brand-new, high-speed HDMI cable (Monoprice Select Series) in a 3ft length - you'll need to make sure whatever you buy is a "high speed" HDMI cable to go higher than 1920x1080.  This was the only way I was able to get a stable output at 2560x1440.  None of the other adapters I've tried (including Anker and Monoprice) were stable at this resolution.  There are reports that "Redmere" super-slim active HDMI cables don't work well with the MacBook, but I didn't have any problems with the one that I tried.

Something else that's a recurring issue with my MacBook is that I have to blow the dust out of the USB-C port before each use - e.g., using canned air - because if I don't, the display is glitchy the way you've described.

So my recommendation (keep in mind what you've paid for it!) is:

1. Try a TV or another monitor that supports 1920x1080 resolution and see if the video display is stable.

2. Try blowing dust out of USB-C port with canned air, giottos rocket, etc.

3. Check your HDMI cable and make sure it's a high speed cable, or just get a new one like the Monoprice Select that you know is high-speed.

4. See if you can borrow/try an Apple Digital Multiport adapter.

Hope this helps.

I'm 0 for four with non-Apple multiport adapters - I've tried the Vic Tsing, Anker, VTin, and Monoprice versions, and none of them could give me a solid, non-glitchy display.  I also gave up and bought the Apple one, and it's been pretty much trouble-free (other than the dust issue I mentioned above).