That's what led me here - should have linked back - seemed this new clean thread might allow it to get discovered and resolved on its own schedule with beta
It's on my list. So is missing toolbar on MacOS, also mentioned the the beta test thread. Probably not technically the same issue, but both likely to be addressed looking into the toolbar details (which really haven't changed since the very early days of Teensy).
Just to add a bit more info, Teensy Loader is built using wxwidgets for all the GUI stuff. Internally these buttons are created from PNG files that get built into the code and converted to a wxwidgets abstraction class for images, and then used in wxwidgets platform independent toolbar class.
Over the years wxwidgets has changed quite a lot, though they generally try to keep it compatible. The code in Teensy Loader for the GUI was written in the late wxwidgets 1.0 days. My guess is we're probably depending on some old API that doesn't give fully featured toolbar anymore. Could also be an issue with how the PNG library is built or initialized. Or the modern toolbar might be expecting one of the newer wxwidgets image container/group classes? I really don't know yet.
It's going to take some fiddling and diving into wxwidgets to learn why the toolbar isn't looking good anymore.