If I go into Config Editor and set PixelsPerPx and set it to 1.0, the "Chrome" (Icon size out of the calendar area) becomes tiny in my current resolution of 3840x2860, which I know seems "oddball" with screen magnfication at the recommended 300%.

However setting it to anything else will result in the magnification and font size settings in the application menu simply not working or doing bizarre things.

"new meeting" is small enough to be unreadable. Seems to not honor the "View|Font Size" increases that the rest ("non-chrome") of the layout does. Also, sometimes the needed controls are not visible (OK/Cancel/Save/etc.) when I do increase PixelsPerPx to say 1.2. Tbird ver 115.9 supernova; Windows 11

I suggest using the size settings on the application menu and forgetting you ever heard of PixelsPerPx once you reset it to default.

I know the problem is mostly Windows-based, because the resolution, scale, and various HDR settings don't seem to be a "thing" when I'm using Thunderbird 105.x in Ubuntu distros such as 22.04 LTS. But Windows will often auto-detect a display mode (after updates/reboots?) that seems to make Thunderbird fall back to defaults, i.e., become unreadable, when I follow your guidance and leave pixelsperpx at 1.0 in the "Config Editor" settings and do all adjustments via the View Menu. The controls I use are Density, Zoom, and Font Size from the View Menu.

Is there a way to make the Thunderbird config file read-only (this seems necessary as mentioned above in the Windows flavor) once I have all setting in a "senior-eyesight-friendly" config? I get that update cadence is pretty frequent, and the read-only mode will likely be reversed in updates, but we are talking sanity here. FWIW, I'm an Outlook refugee, and would hate to go back, but maybe it's the only path for my Windows use, as I've become accustomed to "set and forget" being the mantra for pretty much all configurations regardless of "native" screen size and device display "peculiarities." I'm sure my situation is an "edge case", but what about having Thunderbird allowing "Ctrl-MouseWheel" or "Pinch Zooming" that is an option in many apps to adjust scaling and address layout issues?