Fix: Round up deltas for smooth scrolling, so target will be reached

2 files changed, 19 insertions, 2 deletions

diff --git a/src/core/math_func.hpp b/src/core/math_func.hpp
index df9142462..4a19033f4 100644
--- a/src/core/math_func.hpp
+++ b/src/core/math_func.hpp
@@ -346,6 +346,23 @@ static inline int RoundDivSU(int a, uint b)
 	}
 }
 
+/**
+ * Computes (a / b) rounded away from zero.
+ * @param a Numerator
+ * @param b Denominator
+ * @return Quotient, rounded away from zero
+ */
+static inline int DivAwayFromZero(int a, uint b)
+{
+	const int _b = static_cast<int>(b);
+	if (a > 0) {
+		return (a + _b - 1) / _b;
+	} else {
+		/* Note: Behaviour of negative numerator division is truncation toward zero. */
+		return (a - _b + 1) / _b;
+	}
+}
+
 uint32 IntSqrt(uint32 num);
 
 #endif /* MATH_FUNC_HPP */
diff --git a/src/viewport.cpp b/src/viewport.cpp
index da4ed2660..5e72aaa23 100644
--- a/src/viewport.cpp
+++ b/src/viewport.cpp
@@ -1813,8 +1813,8 @@ void UpdateViewportPosition(Window *w)
 			if (_settings_client.gui.smooth_scroll) {
 				int max_scroll = ScaleByMapSize1D(512 * ZOOM_LVL_BASE);
 				/* Not at our desired position yet... */
-				w->viewport->scrollpos_x += Clamp(delta_x / 4, -max_scroll, max_scroll);
-				w->viewport->scrollpos_y += Clamp(delta_y / 4, -max_scroll, max_scroll);
+				w->viewport->scrollpos_x += Clamp(DivAwayFromZero(delta_x, 4), -max_scroll, max_scroll);
+				w->viewport->scrollpos_y += Clamp(DivAwayFromZero(delta_y, 4), -max_scroll, max_scroll);
 			} else {
 				w->viewport->scrollpos_x = w->viewport->dest_scrollpos_x;
 				w->viewport->scrollpos_y = w->viewport->dest_scrollpos_y;