External monitor doesn't display in full screen

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Level 1

12 points

External monitor doesn't display in full screen

Hi,

I've recently bought an ultrawide 34" monitor and when it's connected to my 13" MacBook Pro, the image doesn't fit the whole screen fo my monitor. Theres a gap of more than a cm around the displayed content. But, when the monitor is connected to my 15" MB Pro, it works fine and displayed content fills the screen!

13" MB Pro details:

MacBook Pro (13-inch, 2017, Two Thunderbolt 3 ports)

2.3 GHz Dual-Core Intel Core i5

Intel Iris Plus Graphics 640 1536 MB

OS: Catalina (up-to-date)

15" MB Pro details:

MacBook Pro (15-inch, 2018)

2.2 GHz Intel Core i7

Radeon Pro 555X 4 GB Intel UHD Graphics 630 1536 MB

OS: Mojave

External monitor

LG Ultrawide 34WL50S-B 34-Inch IPS Monitor (2560 x 1080)

Tried lot of things – changing settings on monitor, on laptop, fiddling with resolutions, using SwitchResX – but nothing works.

Any ideas or is my 13" just not able to output to the screen? Doesn't make sense cos the resolutions seems to support the same on both my MB Pros (2560 x 1080).

Thanks!

MacBook Pro

Posted on Apr 18, 2020 1:49 AM

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Answer 1

Top-ranking reply

Grant Bennet-Alder

Level 10

133,279 points

Apr 18, 2020 9:22 AM in response to mocha_neko

To support "heartbeat-refresh" displays such as your HDMI display or the built-in display, each row of display data must be presented at the exact correct time for display, with strict timing requirements, else that row will be blanked on the display for one refresh period. This repeats for every row of pixels displayed, and repeats again every refresh period, typically 60 times a second.

Excessive blanking will eventually show up as flickering, and be unpleasant to use, whether the flickering is visibly detectable or not.

Your 15-in MacBook Pro uses its AMD discrete graphics subsystem when you connect an external display. That subsystem can more easily support "heartbeat-refresh" displays such as the built-in display or an external HDMI display because it has 4GB or more of special-purpose display RAM (not shared as System RAM).

When using the 13-in MacBook Pro, its display support is provided by the Intel Integrated graphics subsystem, which shares system RAM for the display buffer. The memory bandwidth required for large external HDMI or other "heartbeat-refresh" display may exceed the ability of that RAM to provide a flicker-free display. When the built-in display is used at the same time, the requirements are even more extreme.

 Menu > About this Mac > (System Report) > Graphics&Displays ...

... will show you the ACTUAL achieved resolution, and directly underneath, the "User-Interface looks like" resolution being used by your display.

In almost all cases, using DisplayPort family displays (or thunderBolt, or USB-C) can achieve higher resolutions because the "heartbeat-refresh" is not provided for such displays.

In some cases, a higher resolution for the external display is achievable by using closed display mode:

Use your Mac notebook computer in closed-display mode with an external display - Apple Support

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Answer 2

Apr 18, 2020 6:57 AM in response to mocha_neko

Thanks for attaching some detailed pictures so that it’s easier to diagnose the problem.

try going to system preferences>display and under optimise for click the monitor name in the drop down. It will blank out for a while but it will come back after a few seconds. It should look better now.

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Answer 3

mocha_neko Author

Level 1

12 points

Apr 18, 2020 7:33 AM in response to orangegrouptech

Thanks for the reply and suggestion @orangegrouptech.

Unfortunately that didn't do the trick.

I'm not using the Mirror Displays option, but even with this enabled and optimised for the external monitor, the issue is still unresolved.

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Answer 4

mocha_neko Author

Level 1

12 points

Apr 18, 2020 6:04 PM in response to Grant Bennet-Alder

Thanks heaps @Grant! This is what I was looking for and at least now I know what the issue is.

Unfortunately the close-lid approach doesn't solve the issue as well. Setting a higher refresh rate seems to make the image sharper (at 75 Hertz).

I'll have to stick with what I've got. It's too late to get a new monitor that supports USB-C.

Thanks again and I hope this helps anyone else who comes across this problem.

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External monitor doesn't display in full screen

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