exo: Revert scaling of scroll events

In CL:1993062 this was changed in an attempt to make the smallest
non-zero scroll value aura would give us line up with the minimum
scroll value legacy linux apps could handle. After further discussion
off-line we've decided this is incorrect. Instead, we will keep the
old scale and just adjust the size of small scroll events sent to
linux apps. This CL does the first half of this, the crostini specific
parts will be done in sommelier.

Bug: 1039157, b/162264906
Change-Id: I5a3b26a520ee085a063a71b22c89c877d9734c34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315667
Commit-Queue: Fergus Dall <sidereal@google.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792104}

components/exo/wayland/wayland_pointer_delegate.cc

@@ -96,25 +96,17 @@ void WaylandPointerDelegate::OnPointerButton(base::TimeTicks time_stamp,
 void WaylandPointerDelegate::OnPointerScroll(base::TimeTicks time_stamp,
                                              const gfx::Vector2dF& offset,
                                              bool discrete) {
-  // Values here determined by experiment.
-
-  // We treat 16 units as one mouse wheel click instead of using
-  // MouseWheelEvent::kWheelDelta because that appears to be what aura actually
-  // gives us.
-  constexpr int kAuraScrollUnit = 16;
-  // The minimum scroll from a mouse wheel needs to be a multiple of 5 units
-  // because many Linux apps (e.g. VS Code, Firefox, Chromium) only allow
-  // scrolls to happen in multiples of 5 units. We pick 5 here where Weston
-  // chooses 10 both to more closely match what X apps do, and because unlike
-  // Weston we apply scroll acceleration to the mouse wheel. This means users
-  // can easily scroll large distances even with the smaller minimum unit, while
-  // the smaller base unit allows for smaller scrolls to happen at all.
-  constexpr int kWaylandScrollUnit = 5;
-
-  // The ratio between the above two values. Multiplying by this converts from
-  // aura units to wayland units, dividing does the reverse.
-  constexpr double kAxisStepDistance = static_cast<double>(kWaylandScrollUnit) /
-                                       static_cast<double>(kAuraScrollUnit);
+  // The unit aura considers to be "one scroll tick".
+  const int kAuraScrollUnit = ui::MouseWheelEvent::kWheelDelta;
+
+  // Weston, the reference compositor, treats one scroll tick as 10 units, with
+  // no acceleration applied.
+  constexpr int kWaylandScrollUnit = 10;
+
+  // The ratio between the wayland and aura unit sizes. Multiplying by this
+  // converts from aura units to wayland units, dividing does the reverse.
+  const double kAxisStepDistance = static_cast<double>(kWaylandScrollUnit) /
+                                   static_cast<double>(kAuraScrollUnit);
 
   if (wl_resource_get_version(pointer_resource_) >=
       WL_POINTER_AXIS_SOURCE_SINCE_VERSION) {
@@ -131,12 +123,20 @@ void WaylandPointerDelegate::OnPointerScroll(base::TimeTicks time_stamp,
   if (wl_resource_get_version(pointer_resource_) >=
           WL_POINTER_AXIS_DISCRETE_SINCE_VERSION &&
       discrete) {
+    // Ensure that we never round the discrete value down to 0.
+    int discrete_x = static_cast<int>(x_value / kWaylandScrollUnit);
+    if (discrete_x == 0 && x_value != 0) {
+      discrete_x = copysign(1, x_value);
+    }
+    int discrete_y = static_cast<int>(y_value / kWaylandScrollUnit);
+    if (discrete_y == 0 && y_value != 0) {
+      discrete_y = copysign(1, y_value);
+    }
+
     wl_pointer_send_axis_discrete(
-        pointer_resource_, WL_POINTER_AXIS_HORIZONTAL_SCROLL,
-        static_cast<int>(x_value / kWaylandScrollUnit));
-    wl_pointer_send_axis_discrete(
-        pointer_resource_, WL_POINTER_AXIS_VERTICAL_SCROLL,
-        static_cast<int>(y_value / kWaylandScrollUnit));
+        pointer_resource_, WL_POINTER_AXIS_HORIZONTAL_SCROLL, discrete_x);
+    wl_pointer_send_axis_discrete(pointer_resource_,
+                                  WL_POINTER_AXIS_VERTICAL_SCROLL, discrete_y);
   }
 
   SendTimestamp(time_stamp);