A bar for the drift, and a lighter touch sideways

A moon has no air, so a sideways drift never stops by itself and stopping
one means cancelling the velocity EXACTLY. That is not hard to do; it is
hard to do blind, which is what it was - a number nothing on the screen
said anything about.

So sprite one is a bar whose width is the drift. It runs right from the
middle of the screen for a rightward one and left for a leftward one, so
which way is as plain as how fast, and stopped is the one state with
nothing drawn at all. The whole of it is a target width written once a
frame; the device stretches one tile into it and the program draws
nothing.

Sideways thrust is one a tick rather than two. At two, the smallest
correction available was twice the size it needed to be and overshooting
was the normal outcome.

WHICH ZERO MEANS NOTHING TURNED OUT TO MATTER. A target width of nought is
the NATURAL width, not an empty sprite - so a bar with no drift in it came
out eight pixels wide, sitting at the middle of the screen, saying
"stopped" in the same shape it says "drifting slightly". What draws
nothing is a SIZE of nought, which is the other zero in the other byte.
Both meanings are deliberate and documented and it still caught me out
inside a week of writing them down.

The check for it earned its place by failing on that before it was found,
which is the best evidence a check can offer.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW

Programs/CosmOS/README.md

@@ -726,7 +726,7 @@ from every assembly file in it. Several are old programs written for the bare ma
 | Break | Stops itself twice with SWI osBreak, so that the registers can be seen changing between one stop and the next. |
 | Grid | The first program to use the screen as a screen. Redefines a tile above the font, fills all 128 map rows, and scrolls it diagonally a pixel at a time. |
 | Sprite | Moves a ball across the shell's own text, writing not one byte of the map to do it. It leaves the sprite in the table on the way out, because clearing them is the system's job - see below. |
-| Lander | Lunar Porter, rung one: a lander over a moon that wraps. Flown with a controller if there is one - a held thruster burns every tick it is held for - and with the arrow keys if there is not, where one press is one burn and that is the most the console can say. |
+| Lander | Lunar Porter, rung one: a lander over a moon that wraps. Flown with a controller if there is one - a held thruster burns every tick it is held for - and with the arrow keys if there is not, where one press is one burn and that is the most the console can say. A bar at the bottom is the sideways drift, drawn as a sprite stretched to the speed - a moon has no air, so a drift never stops by itself and stopping one means cancelling it exactly, which is hard to do blind. |
 | Depth | Four pillars at four distances and a ball walking past them, behind the near ones and in front of the far ones. The ball is sprite nought and every pillar is numbered after it, so table order puts it in front of all four - what actually decides is the depth buffer, asked a column at a time. |
 | Flip | Draws a whole screen into the bank that is not being shown, waits, and then shows it in one byte out of one port. It writes nothing else at all - not a tile, not a colour - so it does not ask for the screen to be saved, and the line it printed is still there when it comes back. It deliberately does not put the displayed screen back either, because that is the system's to restore: a program that faulted while flipped could not have. |
 | Edit | A line editor. |