I'm using v5.0.20181.13826 for .NET 4.6.1. I just upgraded today. I have a 4k display running at 3840 x 2160. I have the font scaling set to 200%. The previous version of Fiddler worked with these settings. I accepted the update when prompted today, but the Fiddler UI is no longer scaling the font. Many of the menus are inaccessible or unusable.
I set my resolution to 1920 x 1080 with no scaling and the Fiddler UI was accessible. I was able to access the menus and set the Fiddler font to 16, and returned to my regular 4k display settings. Most of Fiddler was readable under this setting, but several areas don't appear to acknowledge this change. Screen shots attached.