Fix #5366, #5975, taken from @dannyBies

src/definitions/modules/dropdown.js

@@ -3216,6 +3216,7 @@ $.fn.dropdown = function(parameters) {
             ;
             calculations = {
               context: {
+                offset    : $context.offset(),
                 scrollTop : $context.scrollTop(),
                 height    : $context.outerHeight()
               },
@@ -3228,8 +3229,8 @@ $.fn.dropdown = function(parameters) {
               calculations.menu.offset.top += calculations.context.scrollTop;
             }
             onScreen = {
-              above : (calculations.context.scrollTop) <= calculations.menu.offset.top - calculations.menu.height,
-              below : (calculations.context.scrollTop + calculations.context.height) >= calculations.menu.offset.top + calculations.menu.height
+              above : (calculations.context.scrollTop) <= calculations.menu.offset.top - calculations.context.offset.top - calculations.menu.height,
+              below : (calculations.context.scrollTop + calculations.context.height) >= calculations.menu.offset.top - calculations.context.offset.top + calculations.menu.height
             };
             if(onScreen.below) {
               module.verbose('Dropdown can fit in context downward', onScreen);
@@ -3882,4 +3883,4 @@ $.fn.dropdown.settings.templates = {
 
 };
 
-})( jQuery, window, document );
+})( jQuery, window, document );