[mutter] backends/native: Handle triple resolution mouse wheels better

[Marge Bot <marge-bot src gnome org>]

commit c77e943ce21b3bca4f1284a576537ada8e6a2375
Author: Pascal Nowack <Pascal Nowack gmx de>
Date:   Thu Feb 11 12:33:26 2021 +0100

    backends/native: Handle triple resolution mouse wheels better
    
    When a remote desktop user emits a virtual smooth scrolling event, a
    smooth scroll event, that is not emulated, is emitted and on occasion
    a discrete scroll event, that is emulated, is emitted.
    As base for the discrete scrolling event, the smooth scrolling steps
    are accumulated.
    When the accumulated smooth scrolling steps surpass the
    DISCRETE_SCROLL_STEP, the discrete scrolling event is emitted.
    
    Currently, mutter uses for DISCRETE_SCROLL_STEP the value 10, which is
    a terrible value to work with, especially for high resolution mouse
    wheels.
    When a triple resolution mouse wheel is used, each scrolling step will
    have the value 3 1/3.
    Three of such events won't however surpass the DISCRETE_SCROLL_STEP.
    
    To fix this situation, add DBL_EPSILON to the calculation step, when
    checking for the discrete scroll event to ensure that 3 smooth scroll
    events, with each having the value 3 1/3, emit a discrete scrolling
    event.
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1727>

 src/backends/native/meta-seat-impl.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/src/backends/native/meta-seat-impl.c b/src/backends/native/meta-seat-impl.c
index 25608e436d..b2c5e63e5f 100644
--- a/src/backends/native/meta-seat-impl.c
+++ b/src/backends/native/meta-seat-impl.c
@@ -818,8 +818,10 @@ check_notify_discrete_scroll (MetaSeatImpl       *seat_impl,
 {
   int i, n_xscrolls, n_yscrolls;
 
-  n_xscrolls = floor (fabs (seat_impl->accum_scroll_dx) / DISCRETE_SCROLL_STEP);
-  n_yscrolls = floor (fabs (seat_impl->accum_scroll_dy) / DISCRETE_SCROLL_STEP);
+  n_xscrolls = floor ((fabs (seat_impl->accum_scroll_dx) + DBL_EPSILON) /
+                      DISCRETE_SCROLL_STEP);
+  n_yscrolls = floor ((fabs (seat_impl->accum_scroll_dy) + DBL_EPSILON) /
+                      DISCRETE_SCROLL_STEP);
 
   for (i = 0; i < n_xscrolls; i++)
     {