Fix bug not re-calculating scrollbar width when opening modal.

If opening a modal when a scrollbar is present, closing it, then
resizing the window so the scrollbar disappears, then opening the
modal again, the content would shift. This fixes the problem by
always calculating the scrollbar width every time a modal is opened.

Closes #14357.

js/modal.js

@@ -207,8 +207,7 @@
   }
 
   Modal.prototype.checkScrollbar = function () {
-    if (document.body.clientWidth >= window.innerWidth) return
-    this.scrollbarWidth = this.scrollbarWidth || this.measureScrollbar()
+    this.scrollbarWidth = this.measureScrollbar()
   }
 
   Modal.prototype.setScrollbar = function () {
@@ -221,6 +220,7 @@
   }
 
   Modal.prototype.measureScrollbar = function () { // thx walsh
+    if (document.body.clientWidth >= window.innerWidth) return 0
     var scrollDiv = document.createElement('div')
     scrollDiv.className = 'modal-scrollbar-measure'
     this.$body.append(scrollDiv)