Fullscreen not working as expected

Development Programming & Scripting
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1

I am testing the game package in macOS 26.4. It is built using the Unreal Engine 5.7.4.

I am using a MacBook Pro M3 with a 4K external monitor.

By default, the game was set to “Windowed Fullscreen“ mode with “1920 × 1080“ resolution. It performs well, except it is not very clear to render 1080p on a 4K monitor.

  1. I changed the resolution to “3840 x 2160“. Nothing has changed because it is still using “Windowed Fullscreen“ mode.
  2. I changed to “Fullscreen“ mode. It is very clear. However, the FPS is very low. Also noticed the mouse cursor is only half the size. There was a black bar on the top left corner and a green bar on the top right corner. I have never seen them before.
  3. I changed to “Windowed“ mode. It became a window, but I can see only a quarter of the screen. It seems the window size became 8K, but the resolution remains 4K, which results in a low-resolution feel.
  4. I clicked the green button in the top left corner to maximise the window. Now it’s running in 4K fullscreen, although it’s still showing “Windowed“ mode. What surprised me is that the resolution is very clear, the mouse cursor is no longer half the size, and the FPS is back to normal.

I have to repeat all the above steps if I want 4K fullscreen with normal FPS performance every time I enter the game. Is this a bug in the Unreal Engine? Or is it something I didn’t set it right?

2

that mac M3 4K thing is macOS fullscreen/metal path — WindowedFullscreen at 1080 stays windowed at that size (so 1080 on 4K looks soft), true Fullscreen at 3840x2160 goes exclusive but FPS halves and cursor half-scale + bars suggest the HiDPI backing scale is wrong (metal layer size vs display size mismatch).

the plugin i share here is Win64-only so it wont fix the mac cursor scaling, but for reference the Windows fix for half-scale cursor is GetDPICorrectedCursorPosition. on mac you’d need to check the native backing factor path. leaving the link for your Windows build — Multi-Monitor Display Controller | Fab