If you use Thunderbird on both a laptop and an external monitor, you might have noticed something frustrating: when you move Thunderbird’s window between screens, your column layout doesn’t stay consistent.
For me, the message list column kept becoming way too narrow on my laptop screen. I wanted Thunderbird to keep my favorite layout – roughly 10 % folders / 30 % message list / 60 % message preview – whether I’m working on my laptop (1920×1080) or my larger external monitor (2560×1440).
After some digging into Thunderbird 140’s internals, I found that the new “Supernova” UI uses CSS Grid with two internal variables:
