fix: a zero-width band fires on equality or not at all, and the text now says which

The earlier correction moved the claim rather than fixing it: four places still
said the float-equality test "never fires on a real-valued signal", and a +/-1
square is a real-valued signal -- a standalone check against a band of [1, 1]
fired 19 times, because `inside` is true at +1 and false at -1, so the signal
changes side on every transition.

All four now state the measured behaviour: an exact float-equality test against
the level, which an ordinary signal never satisfies and a signal sitting at the
level satisfies on every transition. Neither answer is a band, which is the
reason the second spin is clamped an increment away rather than the detector
special-cased -- the same conclusion, now resting on a true premise.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -687,8 +687,11 @@ modes are unchanged, and only the question "did an event happen at this sample"
   because that is the first sample at which the width is known. `<` a width shorter than the
   signal's own pulses is the glitch trigger: nothing but a runt satisfies it.
 - **window** — the signal entering or leaving a band between two levels. A band is two levels,
-  so the panel refuses a zero-width one: it reduces the condition to a float-equality test,
+  so the panel refuses a zero-width one: it reduces the condition to an exact float-equality
-  which on a real-valued signal never fires.
+  test against the level, which an ordinary signal never satisfies and a signal sitting *at*
+  the level satisfies on every transition — a ±1 square against a band of `[1, 1]` fired 19
+  times in a standalone check. Neither answer is a band, which is why the second spin is
+  clamped an increment away rather than the detector special-cased.
 - **dropout** — the *absence* of a crossing for longer than a timeout. One event per outage:
   the condition re-arms only when a crossing returns.
 

src/condition.cpp

@@ -119,8 +119,11 @@ double EventDetector::feed_window(double value, double t) noexcept {
   if (inside == inside_) return kNoEvent;
   inside_ = inside;
   // The event is the *change of side*, so `Rising` reads as "into the band" and `Falling`
-  // as "out of it". A band of zero width reduces the condition to an equality test on a
+  // as "out of it". A band of zero width is not a band: `inside` reduces to an exact
-  // real-valued signal, which is a float-equality test and therefore unusable: the panel refuses one.
+  // float-equality test against the level, so the condition fires on none of an ordinary
+  // signal's samples and on every transition of one that sits exactly at the level -- a
+  // +/-1 square against a band of [1, 1] fired 19 times in a standalone check. Neither
+  // answer is a band, so the panel refuses one.
   const bool fired = (cond_.edge == TriggerEdge::Rising && inside) ||
                      (cond_.edge == TriggerEdge::Falling && !inside) ||
                      cond_.edge == TriggerEdge::Both;

src/help/08-trigger.txt

@@ -34,8 +34,10 @@ share. <b>Arm</b> starts watching; <tt>a</tt> arms and disarms from the keyboard
   behaviour. <b>Pulse width</b> — a crossing and its return, qualified by how long the excursion
   lasted: less than, greater than, within or outside a pair of widths; the event is timed at the
   closing edge. <b>Window</b> — the signal entering or leaving a band between two levels; the
-  panel keeps the two levels at least one increment apart, because a zero-width band reduces the
+  panel keeps the two levels at least one increment apart, because a zero-width band is not a
-  condition to a float-equality test that never fires on a real-valued signal. <b>Dropout</b> —
+  band: the condition becomes an exact float-equality test against the level, which an ordinary
+  signal never satisfies and a signal sitting at the level satisfies on every transition.
+  <b>Dropout</b> —
   the absence of a crossing for longer than a timeout; one event per outage, re-arms when a
   crossing returns.
 • The shape's own rows are the only ones shown; the rest are hidden, so the panel never offers

src/ui.cpp

@@ -6983,9 +6983,10 @@ GtkWidget* build_trigger_panel(App& app) {
   app.trigger_level2_spin = spin(-1.0e12, 1.0e12, 0.01, 4);
   gtk_widget_set_tooltip_text(app.trigger_level2_spin,
                               "the band's other edge; it is kept at least one increment away "
-                              "from the level, because a band of zero width reduces the "
+                              "from the level, because a band of zero width is not a band: the "
-                              "condition to a float-equality test, which on a real-valued "
+                              "condition becomes an exact float-equality test against the level, "
-                              "signal never fires");
+                              "which an ordinary signal never satisfies and a signal sitting at "
+                              "the level satisfies on every transition");
   app.trigger_level2_row = labeled_box("to level", app.trigger_level2_spin);
   gtk_box_append(GTK_BOX(box), app.trigger_level2_row);