Replacing Bootstrap Dropdown with Dropup (Different activity on two near identical implementations)

css, html, javascript, jquery, twitter-bootstrap-3

Solution

It took some basic math, but I managed to figure out what you desired to do. This code changes the bootstrap classes between dropup and dropdown depending on the room available for a normal dropdown.

I calculated this by detracting the height of the button, dropdownmenu and how far the button was scrolled down in the scrollContainer from the height of the scrollContainer. I got the value how much the div was scrolled down by using the buttons offset and detracting the offset from the scrollContainer.

Here is my jQuery (I selected the .playlist class because this was attached to your scrollContainer, but you should replace it by an id or select it by other means):

$(".dropdown, .dropup").click(function(){
 var dropdownClassCheck = $(this).hasClass('dropdown');
 var buttonOffset = $(this).offset().top;
 var scrollboxOffset = $('.playlist').offset().top;
 var buttonHeight = $(this).height();
 var scrollBoxHeight = $('.playlist').height();
 var dropDownButtonHeight = $(this).children('ul').height();
 dropdownSpaceCheck = scrollBoxHeight>buttonOffset-scrollboxOffset+buttonHeight+dropDownButtonHeight; 
 if(dropdownClassCheck && !dropdownSpaceCheck){
  $(this).removeClass('dropdown').addClass('dropup');
 }
 else if(!dropdownClassCheck && dropdownSpaceCheck){
        $(this).removeClass('dropup').addClass('dropdown');
 }
});

A working JSFiddle

Let me know if there are parts of the code that could be improved/done easier or if there are any problems with my solution.

Problem

I'm working on a project over at github pages, which I replace a bootstrap `.dropdown` with `.dropup` if the div's `overflow-y: scroll` will cause the dropdown menu to be cutoff / overflow. You can see the function working properly at this jsfiddle. Notice if you click on the ellipsis icon to the right on the top rows, it will drop down, if you click on the icon on the bottom rows, it will drop up. Now, my actual implementation (github page), the code is exactly the same (below), but it wants to replace all `.dropdown` classes with `.dropup` when opened, including the top-most row which gets cut off, seen in the photo below. I've been struggling with this for a week and can't quite figure it out. I've tried a few different things that I thought fixed it but ended up just being a hack and didn't work on mobile, or replaced some but not all etc. Here is the Javascript / jQuery I'm using, which can be seen in the jsfiddle and my github source here. ``` $(document).on("shown.bs.dropdown", ".dropdown", function () { // calculate the required sizes, spaces var $ul = $(this).children(".dropdown-menu"); var $button = $(this).children(".song-menu"); var ulOffset = $ul.offset(); // how much space would be left on the top if the dropdown opened that direction var spaceUp = (ulOffset.top - $button.height() - $ul.height()) - $('#playlist').scrollTop(); // how much space is left at the bottom var spaceDown = $('#playlist').scrollTop() + $('#playlist').height() - ((ulOffset.top + 10) + $ul.height()); // switch to dropup only if there is no space at the bottom AND there is space at the top, or there isn't either but it would be still better fit if (spaceDown < 0 && (spaceUp >= 0 || spaceUp > spaceDown)) $(this).addClass("dropup"); }).on("hidden.bs.dropdown", ".dropdown", function() { // always reset after close $(this).removeClass("dropup"); }); ``` Edit: To clear up any confusion, here's an example of the behavior without my added `.dropup` function. jsfiddle Notice when you click the last menu item, it opens the menu but requires scrolling. I specifically want to remove the `.dropdown` class and add `.dropup` in this case, so no scrolling is required.

Original source