Fix horizontal scrolling when shift is held (#25320)

* Fix horizontal scrolling when shift is held

Fixes #24710

* Improve comment

src/vs/base/browser/ui/scrollbar/scrollableElement.ts

@@ -225,7 +225,10 @@ export class ScrollableElement extends Widget {
 				[deltaY, deltaX] = [deltaX, deltaY];
 			}
 
-			if (this._options.scrollYToX && !deltaX) {
+			// Convert vertical scrolling to horizontal if shift is held, this
+			// is handled at a higher level on Mac
+			const shiftConvert = !Platform.isMacintosh && e.browserEvent.shiftKey;
+			if ((this._options.scrollYToX || shiftConvert) && !deltaX) {
 				deltaX = deltaY;
 				deltaY = 0;
 			}