Fix horizontal scrolling-into-view with non-fixed gutters

Closes #6436

Author: marijnh

src/display/scrolling.js

@@ -91,14 +91,15 @@ function calculateScrollPos(cm, rect) {
     if (newTop != screentop) result.scrollTop = newTop
   }
 
-  let screenleft = cm.curOp && cm.curOp.scrollLeft != null ? cm.curOp.scrollLeft : display.scroller.scrollLeft
-  let screenw = displayWidth(cm) - (cm.options.fixedGutter ? display.gutters.offsetWidth : 0)
+  let gutterSpace = cm.options.fixedGutter ? 0 : display.gutters.offsetWidth
+  let screenleft = cm.curOp && cm.curOp.scrollLeft != null ? cm.curOp.scrollLeft : display.scroller.scrollLeft - gutterSpace
+  let screenw = displayWidth(cm) - display.gutters.offsetWidth
   let tooWide = rect.right - rect.left > screenw
   if (tooWide) rect.right = rect.left + screenw
   if (rect.left < 10)
     result.scrollLeft = 0
   else if (rect.left < screenleft)
-    result.scrollLeft = Math.max(0, rect.left - (tooWide ? 0 : 10))
+    result.scrollLeft = Math.max(0, rect.left + gutterSpace - (tooWide ? 0 : 10))
   else if (rect.right > screenw + screenleft - 3)
     result.scrollLeft = rect.right + (tooWide ? 0 : 10) - screenw
   return result