select-multiple: Allow to customise how multiple items are shown in the "summary" part

[rewritten]

Is your feature request related to a problem? Please describe.

I have filters with very narrow and shot layout, and some select-multiple allow numerous elements to be shown. I wish I could, instead of showing the rounded items in the textbox, use the classic select-single layout, and customise how the selection is summarised, for instance: "The selected item", or "The selected item + 3", or "4 items selected"

Describe the solution you'd like
A good example (although it depends on bootstrap and jQuery) is https://mdbootstrap.com/docs/standard/extended/multiselect/ , in the "Search" section

Describe alternatives you've considered
I am currently implementing this completely by hand with a <details>, a <summary> with the computed label and the clear icon, and a list of checkbox inputs inside of the details expanded area. For search, a custom input with a lot of event listeners and fuzzysort as a matcher library (because I cannot access the Fuse class that is embedded here)