My initial thought is that I'd personally avoid attaching a chevron to the checkbox like you've got in your screenshot because it's an atypical design pattern and therefore pretty safe to assume that a (possibly significant) percentage of your user base won't be familiar with it and they'll have to work to figure it out, which means increased cognitive load, ...
There is a similar UI for managing exceptions in Visual Studio, which is fairly intuitive:
The key differences to the UI you have shown are:
Chevrons/arrows appear to the left of the top level checkboxes, making the control more like a familiar tree control.
The top level checkboxes are tri-state checkboxes so you can see if only all/some/none of the child ...
When clicking "Backup Media Library", should it select/unselect, or expand/collapse?
It should select/unselect. This is the default behavior and makes the checkbox more accessible.
If the expand/collapse behavior is tied to the chevron alone (my preference), is it too small for the user to click? Should I replace it with something bigger, like [...
Split the configuration into two parts: Conditions and Actions.
Each condition should be on a dedicated line (row). Provide the user buttons (or links) to be able to add additional conditions and remove existing ones.
Subsequent rows (after the first) should give the user the option for AND/OR, and depending on your requirements, you probably want to prevent ...
It sounds like you're looking for Hick's Law: Increasing the number of choices increases the decision time logarithmically.
You might also want to look around at Nielsen Norman, who publish well-researched articles. Here's one on dropdown lists.
In general, lists with multiple elements are usually accompanied by keyboard help. When clicking on a letter, the first element of the list is the one that begins with the letter pressed.
At a graphic level, we have this situation resolved horizontally but not vertically. Although the example is a list whose elements are arranged vertically, one below the ...
I don't think there is a specific answer for the question since it will definitely depend on the context and various requirements or constraints.
However, you can reference examples like the searchable dropdown lists used for tasks like searching for a country or looking up an address.
If the search algorithm and autocomplete feature is smart or powerful ...
Yes. Select controls and drop-down menus are a form of modal interface and should block interaction with the underlying content.
This is often experienced as dismissing the menu or UI element by clicking outside of it when you don't want to make a choice. The underlying stuff that you click on doesn't respond to that click, it only dismisses the open menu ...
For those coming from google: Since iOS 14 there are pull-down menus in iOS.
Here is a link to the guidelines: https://developer.apple.com/design/human-interface-guidelines/ios/controls/pull-down-menus/
See my answer for some basic example code: