Avoid assigning NSCursor style when it already is that style

Julia and Antonio Scandurra created 

This avoids a high cost which appears to be the system rasterizing the
cursor every time we call this, fixes a slowdown when scrolling rapidly
while mouse motion continually attempted to assign the style

Co-Authored-By: Antonio Scandurra <me@as-cii.com>

Change summary

crates/gpui/src/platform/mac/platform.rs | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

Detailed changes

crates/gpui/src/platform/mac/platform.rs 🔗 

@@ -786,7 +786,7 @@ impl platform::Platform for MacPlatform {
 
     fn set_cursor_style(&self, style: CursorStyle) {
         unsafe {
-            let cursor: id = match style {
+            let new_cursor: id = match style {
                 CursorStyle::Arrow => msg_send![class!(NSCursor), arrowCursor],
                 CursorStyle::ResizeLeftRight => {
                     msg_send![class!(NSCursor), resizeLeftRightCursor]
@@ -795,7 +795,11 @@ impl platform::Platform for MacPlatform {
                 CursorStyle::PointingHand => msg_send![class!(NSCursor), pointingHandCursor],
                 CursorStyle::IBeam => msg_send![class!(NSCursor), IBeamCursor],
             };
-            let _: () = msg_send![cursor, set];
+
+            let old_cursor: id = msg_send![class!(NSCursor), currentCursor];
+            if new_cursor != old_cursor {
+                let _: () = msg_send![new_cursor, set];
+            }
         }
     }