SDL: cocoa: Don't round scroll deltas from trackpads

SDL Commits
1 
From da0f76de6d8e31c610a18933423213c46974388e Mon Sep 17 00:00:00 2001
From: Cameron Gutman <[EMAIL REDACTED]>
Date: Tue, 7 Dec 2021 00:38:46 -0600
Subject: [PATCH] cocoa: Don't round scroll deltas from trackpads

Rounding the scroll deltas from trackpads causes jerky scrolling behavior
by artificially amplifying the effects of very small scroll movements.

We should only round events from devices with discrete scroll wheels,
because we know the smallest unit of movement there is a single tick.
---
 src/video/cocoa/SDL_cocoamouse.m | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/src/video/cocoa/SDL_cocoamouse.m b/src/video/cocoa/SDL_cocoamouse.m
index e9d832d6468..9031c83eddd 100644
--- a/src/video/cocoa/SDL_cocoamouse.m
+++ b/src/video/cocoa/SDL_cocoamouse.m
@@ -473,15 +473,19 @@ + (NSCursor *)invisibleCursor
         }
     }
 
-    if (x > 0) {
-        x = SDL_ceil(x);
-    } else if (x < 0) {
-        x = SDL_floor(x);
-    }
-    if (y > 0) {
-        y = SDL_ceil(y);
-    } else if (y < 0) {
-        y = SDL_floor(y);
+    /* For discrete scroll events from conventional mice, always send a full tick.
+       For continuous scroll events from trackpads, send fractional deltas for smoother scrolling. */
+    if (![event respondsToSelector:@selector(hasPreciseScrollingDeltas)] || ![event hasPreciseScrollingDeltas]) {
+        if (x > 0) {
+            x = SDL_ceil(x);
+        } else if (x < 0) {
+            x = SDL_floor(x);
+        }
+        if (y > 0) {
+            y = SDL_ceil(y);
+        } else if (y < 0) {
+            y = SDL_floor(y);
+        }
     }
 
     SDL_SendMouseWheel(window, mouseID, x, y, direction);