G6: a wheel's notch scrolls the core at once and the picture glides after it

A mouse wheel's notch (SDL's whole-number step, notchOf) scrolls the core
by its lines at once, so keys and clicks act where the text is, and the
picture follows on the Motion flavour's spring at half again its pace,
critically damped whatever the flavour (text never passes its mark),
carried from its current speed (animation.Spring.carry), so notches in a
row flow on. How far the rows moved is measured on the next frame against
the last (Glide.measure: a wrapped line's rows, a terminal's output, with a
couple of rows' slack for the cursor's line and its number); the rows that
left are kept, nearest first, and drawn in the gap the lag opens; the lag
is never more than the body's height. A touchpad's fractions move the
picture under the finger as before; `Motion off` lands a notch at once;
a PDF page scrolls as before.

The glide runs on the core's continuous path: Pardes.shell_continuous
keeps frames coming every loop while it moves and carries a virtual clock
with it; idle, nothing. The test feed takes wheel events
(ESC]777;mouse;wheel;<amount>;<x>;<y>BEL).

Tests: notch vs fraction; a carried spring keeps its speed and never
passes its mark; a notch over two-row lines lags two rows, keeps the rows
that left, settles frame by frame and leaves the continuous path, and
lands at once with Motion off. Gates as G5. Feel review:
.scratch/render/scroll/glide-{smooth,crisp,bouncy,off}{,-quarter}.mp4 and
strips (a notch down, three, then two up).

1 files changed, 18 insertions, 0 deletions

diff --git a/docs/effects.md b/docs/effects.md
index 8a60d116..04757aca 100644
--- a/docs/effects.md
+++ b/docs/effects.md
@@ -39,6 +39,7 @@ why. The verdict is the user's.
 | G3 cursor | follows `Motion`; `CursorBlink on\|off` (on) | glide on by default through the default flavour (smooth); blink on (decision 6) | .scratch/render/cursor/cursor-flavours-grid.mp4 (and quarter speed), raw/<flavour>/<motion>/ | pending |
 | G4 bloom, vignette, grain | `Bloom`, `Vignette`, `Grain` `0..3` (off) | bundled post passes; none touches a tag, grip, notice or the cursor (pardesSpare); each capped per theme (pardesCap) | .scratch/render/{bloom,vignette,grain}/ (levels 0-3 on forge, dusk, acme; crops; mp4 stepping through the levels) | pending |
 | G5 lapis | `Theme lapis` (its `decor`) | a native theme with ornament: tag plaques with hard shadows inside their bands (the focused one striped), dots, rail checker, title shadow | .scratch/render/lapis/ (whole window at 1x and 2x, crops, a slide mp4) | pending |
+| G6 smooth scroll | follows `Motion` (on through the default smooth; `Motion off` lands at once) | a mouse wheel's notch glides, a touchpad follows the finger | .scratch/render/scroll/ (glide-<flavour>.mp4, quarter speed, strips) | pending |
 
 ## Motion flavours
 
@@ -143,3 +144,20 @@ file name's letters. Left out: Crimson Pro and Departure Mono, the
 ornaments, a decor crossfade (it snaps while the colours fade). The tty
 shows the colours alone: a tag row abuts its body and its neighbour, so
 there is no cell for a shadow row or column.
+
+Notes on G6: a mouse wheel's notch (SDL's whole-number step) scrolls the
+core by its lines at once, so keys and clicks act where the text is, and
+the picture follows on the Motion flavour's spring at half again its pace,
+critically damped whatever the flavour (text never passes its mark; §8.1),
+from its current speed, so notches in a row carry on without a stall. How
+far the rows moved is measured on the next frame against the last (a
+wrapped line's rows, a terminal's output), within a couple of rows' slack
+for the cursor's line and its number moving with the scroll; the rows that
+left are kept and drawn in the gap the lag opens, and a lag is never more
+than the body's height (a flick past that jumps). A touchpad's fractions
+keep moving the picture under the finger, as before; there is no momentum
+(SDL reports no finger lift on a wheel). The glide runs on the core's
+continuous path (Pardes.shell_continuous), so the shell draws every loop
+while it moves and a virtual clock moves with it; idle, it costs nothing.
+A PDF page scrolls as before. The test feed takes wheel events
+(ESC]777;mouse;wheel;<amount>;<x>;<y>BEL).