I want to build a Web app with SvelteKit with one page listing all items (with potential search query parameters), and then one page for each individual item. If I had to build this the old school way with everything generated in the backend, my paths would be /items/ for the list of all items, and /items/123 for item 123, etc. That is, to go to the page of item 123, a link will with href="123" will work no matter if you are currently at the index (/items/) or at the page of one particular item (/items/[id]).
With SvelteKit, if I create files routes/items/index.svelte and routes/items/[id].svelte, then routes/items/index.svelte will have path /items, without a trailing slash, and as a result a link with href="123" will lead to /123, resulting in a "not found" error.
This same link will work however from the page of an individual item, say, /items/456.
This is radically different from what you would have in the traditional HTML model, where a link from /items/ (or /items/index.html) would work the same as a link from /items/[id].html.
Now in svelte.config.js there is a trailingSlash option you can set to always so that routes/items/index.svelte corresponds to path /items/, but then routes/items/[id].svelte has path /items/[id]/ and we have the same problem again: one href value cannot work from both the index and the page of an individual item.
The only way I see right now is to use absolute path, but it's not very composable. My guess is that there is something I am doing wrong.
You're not missing anything - it's not currently possible in SvelteKit to have a trailing slash for some pages but not for others. There is an open GitHub issue you may be interested in that proposes adding additional trailingSlash options. This issue cites the exact problem you described:
The trailingSlash options introduced in #1404 don't make it straightforward to add trailing slashes to index pages but not to leaf pages. For example you might want to have /blog/ and /blog/some-post rather than /blog and /blog-some-post, since that allows the index page to contain relative links.
Until that feature is added, you'll need to use absolute paths.