When your list reaches 20 items or more, SharePoint decided to automatically replace this simple, clean, and elegant dropdown with some messy, sloppy, and difficult-to-use Complex Dropdown. This is meant to be an upgrade. The text box, with jQuery behind it, allows users to type the item that they're looking for (which you can actually already do with a dropdown). However, in practice, it is prone to unexpected behavior:
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I have a requirement to cascade country and state dropdown in SharePoint list. I have tried it using SPCascadeDropdowns and it's not working more than 20 items in child dropdowns. This states my child dropdowns:
If set to true, the child dropdown will be converted to a "simple" dropdown - only if it is a "complex" dropdown on page load. Edit the page in the browser and add a Content Editor Web part, modify the web part and show the Source Editor to add the following code.:
We made the cascading lookup to display more than 20 items successfully. The lookup column now works fine for more than 20 entries also.