Handle lack of .width and .height from getBoundingClientRect() in IE8

Closes #14093
Correction to #14090
Relevant docs: https://developer.mozilla.org/en-US/docs/Web/API/Element.getBoundingClientRect#Browser_compatibility

js/tooltip.js

@@ -324,6 +324,17 @@
     var isSvg  = window.SVGElement && el instanceof window.SVGElement
 
     var elRect    = el.getBoundingClientRect()
+    if (elRect.width == null) {
+      // width and height are missing in IE8, so compute them manually; see https://github.com/twbs/bootstrap/issues/14093
+      elRect = {
+        bottom: elRect.bottom,
+        top:    elRect.top,
+        right:  elRect.right,
+        left:   elRect.left,
+        width:  elRect.right  - elRect.left,
+        height: elRect.bottom - elRect.top
+      }
+    }
     var elOffset  = isBody ? { top: 0, left: 0 } : $element.offset()
     var scroll    = { scroll: isBody ? document.documentElement.scrollTop || document.body.scrollTop : $element.scrollTop() }
     var outerDims = isSvg ? {} : {