a mostly-vertical touchpad swipe stays vertical

The pad faithfully turns a two-finger scroll's sideways drift into wheel_left
and wheel_right, so a plain scroll slid the view sideways underneath you. Every
vertical tick now re-arms a guard and every horizontal tick spends one instead
of scrolling, so horizontal has to EARN its way back by landing three ticks in
a row with no vertical among them.

Clock-free on purpose: the core is a state machine with no timestamp on a mouse
event, and faking one by counting renders would be worse than the counter. The
guard is only ever armed BY vertical scrolling, so a horizontal swipe from a
still view still moves on its first tick — only horizontal that interrupts
vertical has anything to prove.

A tilt wheel gets the same treatment, where recent-vertical is a much weaker
signal of accident. Deliberate: the only honest fix is a per-device flag out of
the shell, and that layer costs more than the three clicks it would save.

The rule is one pure function next to the number it reads, with an inline test
written to hold for any tuning of that number. unit-test grew a fourth binary
over the core module hxdiff already links. New golden wheeldrift; none moved.

1 files changed, 85 insertions, 0 deletions

diff --git a/test/snapshots/wheeldrift.golden b/test/snapshots/wheeldrift.golden
new file mode 100644
index 00000000..44d40eef
--- /dev/null
+++ b/test/snapshots/wheeldrift.golden
@@ -0,0 +1,85 @@
+== snap home grid=70x16 cursor=7,2
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/wheeldrift/c  NOR /tmp/pardes-snap/wheeldrift/c
+|     1 line 1_0123456789_abcdefghij
+|     2 line 2_0123456789_abcdefghij  drift.txt
+|     3 line 3_0123456789_abcdefghij
+|     4 line 4_0123456789_abcdefghij
+|     5 line 5_0123456789_abcdefghij
+|     6 line 6_0123456789_abcdefghij
+|     7 line 7_0123456789_abcdefghij
+|     8 line 8_0123456789_abcdefghij
+|     9 line 9_0123456789_abcdefghij
+|    10 line 10_0123456789_abcdefghi
+|    11 line 11_0123456789_abcdefghi
+|    12 line 12_0123456789_abcdefghi
+|    13 line 13_0123456789_abcdefghi
+|    14 line 14_0123456789_abcdefghi
+== snap drifted grid=70x16 cursor=7,2
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/wheeldrift/c  NOR /tmp/pardes-snap/wheeldrift/c
+|     6 line 6_0123456789_abcdefghij
+|     7 line 7_0123456789_abcdefghij  drift.txt
+|     8 line 8_0123456789_abcdefghij
+|     9 line 9_0123456789_abcdefghij
+|    10 line 10_0123456789_abcdefghi
+|    11 line 11_0123456789_abcdefghi
+|    12 line 12_0123456789_abcdefghi
+|    13 line 13_0123456789_abcdefghi
+|    14 line 14_0123456789_abcdefghi
+|    15 line 15_0123456789_abcdefghi
+|    16 line 16_0123456789_abcdefghi
+|    17 line 17_0123456789_abcdefghi
+|    18 line 18_0123456789_abcdefghi
+|    19 line 19_0123456789_abcdefghi
+== snap paying grid=70x16 cursor=7,2
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/wheeldrift/c  NOR /tmp/pardes-snap/wheeldrift/c
+|     6 line 6_0123456789_abcdefghij
+|     7 line 7_0123456789_abcdefghij  drift.txt
+|     8 line 8_0123456789_abcdefghij
+|     9 line 9_0123456789_abcdefghij
+|    10 line 10_0123456789_abcdefghi
+|    11 line 11_0123456789_abcdefghi
+|    12 line 12_0123456789_abcdefghi
+|    13 line 13_0123456789_abcdefghi
+|    14 line 14_0123456789_abcdefghi
+|    15 line 15_0123456789_abcdefghi
+|    16 line 16_0123456789_abcdefghi
+|    17 line 17_0123456789_abcdefghi
+|    18 line 18_0123456789_abcdefghi
+|    19 line 19_0123456789_abcdefghi
+== snap moved grid=70x16 cursor=7,2
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/wheeldrift/c  NOR /tmp/pardes-snap/wheeldrift/c
+|     6 123456789_abcdefghij_ZEND
+|     7 123456789_abcdefghij_ZEND     drift.txt
+|     8 123456789_abcdefghij_ZEND
+|     9 123456789_abcdefghij_ZEND
+|    10 0123456789_abcdefghij_ZEND
+|    11 0123456789_abcdefghij_ZEND
+|    12 0123456789_abcdefghij_ZEND
+|    13 0123456789_abcdefghij_ZEND
+|    14 0123456789_abcdefghij_ZEND
+|    15 0123456789_abcdefghij_ZEND
+|    16 0123456789_abcdefghij_ZEND
+|    17 0123456789_abcdefghij_ZEND
+|    18 0123456789_abcdefghij_ZEND
+|    19 0123456789_abcdefghij_ZEND
+== snap rearmed grid=70x16 cursor=7,2
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/wheeldrift/c  NOR /tmp/pardes-snap/wheeldrift/c
+|     5 123456789_abcdefghij_ZEND
+|     6 123456789_abcdefghij_ZEND     drift.txt
+|     7 123456789_abcdefghij_ZEND
+|     8 123456789_abcdefghij_ZEND
+|     9 123456789_abcdefghij_ZEND
+|    10 0123456789_abcdefghij_ZEND
+|    11 0123456789_abcdefghij_ZEND
+|    12 0123456789_abcdefghij_ZEND
+|    13 0123456789_abcdefghij_ZEND
+|    14 0123456789_abcdefghij_ZEND
+|    15 0123456789_abcdefghij_ZEND
+|    16 0123456789_abcdefghij_ZEND
+|    17 0123456789_abcdefghij_ZEND
+|    18 0123456789_abcdefghij_ZEND