[gtk/wip/carlosg/spin-button-swipe-gesture-4-2: 2/2] spinbutton: Mind the step in the swipe gesture

[Carlos Garnacho <carlosg src gnome org>]

commit 6df3296211586c179a8f85f89dcee314d6ab3b9a
Author: Carlos Garnacho <carlosg gnome org>
Date:   Tue Jun 8 15:09:37 2021 +0200

    spinbutton: Mind the step in the swipe gesture
    
    The swipe gesture forces values in the spin button that are
    "impossible" according to the adjustment. This can break things
    in creative ways.
    
    Ensure the steps provided are always multiples of the adjustment
    step value, and keep the remainder for further interaction.

 gtk/gtkspinbutton.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtkspinbutton.c b/gtk/gtkspinbutton.c
index ba7a3d5fa9..7f59b741b0 100644
--- a/gtk/gtkspinbutton.c
+++ b/gtk/gtkspinbutton.c
@@ -208,6 +208,7 @@ struct _GtkSpinButton
 
   double         climb_rate;
   double         timer_step;
+  double         swipe_remainder;
 
   int            width_chars;
 
@@ -838,6 +839,7 @@ swipe_gesture_begin (GtkGesture       *gesture,
 {
   gtk_gesture_set_state (gesture, GTK_EVENT_SEQUENCE_CLAIMED);
   gtk_widget_grab_focus (GTK_WIDGET (spin_button));
+  spin_button->swipe_remainder = 0;
 }
 
 static void
@@ -845,10 +847,12 @@ swipe_gesture_update (GtkGesture       *gesture,
                       GdkEventSequence *sequence,
                       GtkSpinButton    *spin_button)
 {
-  double vel_y;
+  double vel_y, step;
 
   gtk_gesture_swipe_get_velocity (GTK_GESTURE_SWIPE (gesture), NULL, &vel_y);
-  gtk_spin_button_real_spin (spin_button, -vel_y / 20);
+  step = (-vel_y / 20) + spin_button->swipe_remainder;
+  spin_button->swipe_remainder = fmod (step, gtk_adjustment_get_step_increment (spin_button->adjustment));
+  gtk_spin_button_real_spin (spin_button, step - spin_button->swipe_remainder);
 }
 
 static gboolean