input: slow trackpad scrolling now eventually scroll a line

Before, we converted each axis event's scroll amount to an integer and
scrolled that many lines.

However, axis events are speed sensitive - very slow scrolling will
result in events with a scroll amount that is < 1.0.

For us, this meant we never scrolled a single line. You could slow
scroll all day if you wanted, and still we would never scroll a single
line.

Fix this by aggregating the scroll amount from axis events until the
scroll amount is > 1.0, and then scroll.

CHANGELOG.md

@@ -48,6 +48,7 @@
 * Hostname in OSC 7 URI not being validated.
 * OSC 4 with multiple `c;spec` pairs.
 * Alt+Return to emit "ESC \r".
+* Trackpad sloooow scrolling to eventually scroll a line.
 
 
 ### Security

input.c

@@ -1295,7 +1295,18 @@ wl_pointer_axis(void *data, struct wl_pointer *wl_pointer,
     if (wayl->mouse.have_discrete)
         return;
 
-    mouse_scroll(wayl, wl_fixed_to_int(value));
+    /*
+     * Aggregate scrolled amount until we get at least 1.0
+     *
+     * Without this, very slow scrolling will never actually scroll
+     * anything.
+     */
+    wayl->mouse.axis_aggregated += wl_fixed_to_double(value);
+
+    if (fabs(wayl->mouse.axis_aggregated) >= 1.) {
+        mouse_scroll(wayl, round(wayl->mouse.axis_aggregated));
+        wayl->mouse.axis_aggregated = 0.;
+    }
 }
 
 static void
@@ -1327,6 +1338,11 @@ static void
 wl_pointer_axis_stop(void *data, struct wl_pointer *wl_pointer,
                      uint32_t time, uint32_t axis)
 {
+    if (axis != WL_POINTER_AXIS_VERTICAL_SCROLL)
+        return;
+
+    struct wayland *wayl = data;
+    wayl->mouse.axis_aggregated = 0.;
 }
 
 const struct wl_pointer_listener pointer_listener = {

wayland.h

@@ -292,6 +292,7 @@ struct wayland {
         struct timeval last_time;
 
         /* We used a discrete axis event in the current pointer frame */
+        double axis_aggregated;
         bool have_discrete;
     } mouse;