SUMMARY I'm on arch linux, with a hi-dpi screen, using Noto fonts. When I use https://www.reddit.com/r/archlinux/comments/9q8dlj/how_to_better_enable_color_emojis/ to enable Emoji rendering, Plasma (and only Plasma) renders all text as tiny. This can be seen on the panel, but also in any settings windows that are opened from within Plasma (eg: by right-clicking the panel and selecting "Configure task manager" - note that System Settings does *not* have this problem). Without the above-linked config file, Plasma renders fonts at a sensible size (but doesn't render emoji). I've tried other fontconfig methods to use Noto Color Emoji as a fallback for Noto fonts, and they either don't cause emoji to be rendered, or they cause this same problem of Plasma font sizes (along with other problems). No other applications (GTK+ or Qt/KDE) have this issue. KWin is fine - it's rendering title bars as expected. SOFTWARE/OS VERSIONS Linux/KDE Plasma: Arch Linux (available in About System) KDE Plasma Version: 5.19.0 KDE Frameworks Version: 5.71.0 Qt Version: 5.15.0 ADDITIONAL INFORMATION I have the following Arch Linux packages that affect fontconfig: fontconfig2:2.13.91+24+g75eadca-2 gsfonts20180524-3 noto-fonts20190926-4 noto-fonts-cjk20190409-2 terminus-font4.48-1 texlive-core2019.52579-1 ttf-dejavu2.37-3 ttf-droid20121017-7 ttf-inconsolata1:3.000-2 wine5.10-1 The only other fontconfig change I've made is to add the file from https://www.reddit.com/r/archlinux/comments/9q8dlj/how_to_better_enable_color_emojis/ In system settings, my font configuration is Noto Sans 10pt for everything, except Noto Sans 8pt for "small" and Hack 10pt for "Fixed width". Font DPI is forced to 192.
It may be worth noting that if I disable the "Force font DPI" setting in System Settings, all interface fonts end up the same tiny size as the Plasma ones (HTML rendering in Firefox/Thunderbird/Akregator is unaffected). This happens whether or not I have the emoji fontconfig file enabled. I have the Global Scale set to 200%. It seems possible that Plasma is somehow ignoring the font DPI override when the emoji fontconfig file is present for some reason.
Welp, I was messing around with enabling and disabling the "Force font DPI" and emoji fontconfig stuff in order to generate more details and make some screenshots, and now it's working. I enabled and disabled and re-enabled the fontconfig stuff several times before this, so I think it was disabling and re-enabling the "Force font DPI" setting that resolved it (although I have no idea why, except maybe that I last enabled it several releases ago).