I just tried Decker on a web browser on the phone, and while it works, a couple of seemingly superficial properties make it much less usable than it could be:

1. It wastes a lot of space in the margins, more so than on my large laptop screen. This waste extends on all 4 sides, so it doesn't seem to be just to preserve an aspect ratio.
2. It doesn't use the system (soft) keyboard, and instead splits its already in-use area to show its own keyboard.

I'm curious to learn what the design considerations are that led to the current state. Is there a major obstacle here, do people actually prefer this, or is the small-screen experience just lagging other devices so far?

I think most people prefer to use the native Decker application on a computer to author their projects rather than any kind of web Decker, if the option exists.

But it is possible to reduce the amount of extra space around the program on a phone screen.

If you hold your phone in a horizontal rotation and select Decker > Fullscreen in the menu bar at the top it brings everything in closer.

And optionally you could also select Decker > Toolbars, which makes a variety of drawing tools available in the remaining gaps on the sides.

(Even if you're not drawing anything it can be useful to be able to switch between Interact mode and Widget mode with a single click -- or tap -- with the buttons at the top of the left side toolbar.)

Decker's touch mode is primarily designed with tablets in mind. Some users have reported a good experience on those phones which support a stylus for more precise input.

Decker uses a custom soft keyboard in order to provide a consistent experience on all devices and operating systems in both web-decker and native-decker, with the ability to easily type any of the characters used in Lil scripts. Having fine control over the behavior of this soft keyboard is also important for supporting some functionality.

Various mobile browsers often report misleading or inaccurate information about the screen and its DPI to webapps, including invisibly reserving space for their own UI elements or introducing their own scaling factors. On iPhones specifically, Decker's "fullscreen mode" functionality does not work, because iOS Safari intentionally does not implement the standard fullscreen APIs available on other devices. I try to avoid adding device-specific or browser-specific code to web-decker whenever I can, because that kind of work is inherently brittle.

You could also change the size of your deck to make it more phone friendly, for example going into the .deck file and swapping the length and width values under "size" to enable a more vertical view. Here's an example of someone who did that in the latest jam:

https://erikhoudini.itch.io/the-fish-bowl-get-your-ears-wet

Thanks all for the explanations and tips, full screen mode + toolbars is pretty decent.