From 3a9dbcaaba6c6c34d6b136fa8526530c17c66617 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Emilio=20Cobos=20=C3=81lvarez?= <emilio@crisal.io>
Date: Fri, 13 May 2022 15:59:24 +0000
Subject: [PATCH] Bug 1769269 - Center non-overlay scrollbar thumb on cocoa.
 r=mstange

As per discussion in bug 1764435.

Differential Revision: https://phabricator.services.mozilla.com/D146315
---
 widget/ScrollbarDrawingCocoa.cpp | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/widget/ScrollbarDrawingCocoa.cpp b/widget/ScrollbarDrawingCocoa.cpp
index 3454044bef84..c79911591969 100644
--- a/widget/ScrollbarDrawingCocoa.cpp
+++ b/widget/ScrollbarDrawingCocoa.cpp
@@ -162,11 +162,17 @@ static ThumbRect GetThumbRect(const LayoutDeviceRect& aRect,
   }
   thickness *= aScale;
 
-  // Compute the thumb rect.
-  const float outerSpacing =
-      ((aParams.isOverlay || aParams.isSmall) ? 1.0f : 2.0f) * aScale;
   LayoutDeviceRect thumbRect = aRect;
   thumbRect.Deflate(1.0f * aScale);
+  // Compute the thumb rect. The thumb rect is centered in non-overlay mode,
+  // and spaced in overlay mode.
+  const float outerSpacing = [&] {
+    if (aParams.isOverlay) {
+      return 1.0f * aScale;
+    }
+    float size = aParams.isHorizontal ? thumbRect.Height() : thumbRect.Width();
+    return (size - thickness) / 2.0f;
+  }();
   if (aParams.isHorizontal) {
     float bottomEdge = thumbRect.YMost() - outerSpacing;
     thumbRect.SetBoxY(bottomEdge - thickness, bottomEdge);