Duplicate of #2669.
This is something that I was working on when I have time.

I forgot I already had opened that issue!

@TillerBurr Do you see any difficulties in the implementation? I don't know how far you got already but I could also try to have a look.

@WWakker No worries. It actually wasn't that bad implementing. The only thing that was slightly challenging were the integration tests, mainly making sure the menu stayed open.

I had forgotten that I had started thinking on this a couple weeks ago and this issue reminded me that I should figure out those tests.

Related! #1934

The dropdown should revert to the previous scroll state when reopened, so that you don't have to scroll all the way back down to the position you were. This is especially useful when you have hundreds of dropdown options. The dropdown with multi=False does this, but with multi=True, the scroll always reverts to the top. This is even more convenient than leaving the dropdown open after a selection and should be standard behavior.

A workaround to revert back to the previous scroll position for a dropdown with multi=True is to create a assets/scroll.js file, then put the ids of your dash dropdowns in document.querySelector("#id1,#i2,...") below.

var scrollPos = 0;

document.addEventListener('DOMContentLoaded', () => {
    
    function setup_click_listener() {
        var dropdownContainer = document.querySelector("#dropdown, #dropdown-directionality-measurements"); // put dash dropdown ids here
        if (dropdownContainer == null) { // Try again until the dropdown exists
            setTimeout(setup_click_listener, 200);
            return;
        }
        dropdownContainer.addEventListener('click', function() {
            dropdownMenu = dropdownContainer.querySelector('.Select-menu-outer');
            dropdownGrid = dropdownMenu.querySelector('.ReactVirtualized__Grid');
            dropdownGrid.scrollTop = scrollPos;
            dropdownGrid.onscroll = (event) => {
                scrollPos = dropdownGrid.scrollTop;
            }
        });
    }
    setup_click_listener();
})