Fix hover-tooltip flickering when mouse re-enters

- is(':visible') seems to be the only reliable check, without a refactoring of how hoverState is used
- tests need improvement

js/tooltip.js

@@ -105,6 +105,11 @@
     var self = obj instanceof this.constructor ?
       obj : $(obj.currentTarget).data('bs.' + this.type)
 
+    if (self && self.$tip && self.$tip.is(':visible')) {
+      self.hoverState = 'in'
+      return
+    }
+
     if (!self) {
       self = new this.constructor(obj.currentTarget, this.getDelegateOptions())
       $(obj.currentTarget).data('bs.' + this.type, self)