I installed Libreelec on my Raspberry Pi 3B+ and hooked it up to a 34" 2560x1080px ultra-wide computer screen. To deal with this unusual scenario, I had to define a few things in the config.txt file to get it all ticking. It seems to be working nicely so far.
Here is the info needed to make it work:
#disable_overscan=1 #hdmi_force_hotplug=1 hdmi_ignore_cec_init=1 hdmi_drive=2 hdmi_ignore_edid=0xa5000080 hdmi_group=2 hdmi_mode=87 hdmi_aspect_21_9=7 hdmi_pixel_freq_limit=400000000 hdmi_cvt=2560 1080 60 7 0 0 1 #config_hdmi_boost=8 max_framebuffer_width=2560 max_framebuffer_height=1080 framebuffer_width=2560 framebuffer_height=1080