srcset hydration results in expensive DOM updates

[benmccann]

Describe the bug

Svelte always thinks the client side srcset is different from the server side srcset. For sites that make heavy use of picture (e.g. via a generic Image component like in sveltejs/kit#241 (comment) or sveltejs/kit#9787) this ends up being really expensive.

If /myimg.png is SSR'd, the client-side URL contains the full URL including hostname, so Svelte thinks they don't match. There's special logic for this around an img src attribute, but srcset was overlooked.

svelte/packages/svelte/src/compiler/compile/render_dom/wrappers/Element/Attribute.js

Line 164 in 7bab2d4

b`if (!@src_url_equal(${element.var}.src, ${init})) ${method}(${element.var}, "${name}", ${this.last});`

Reproduction

Any picture with a srcset

Logs

No response

System Info

Happens with 3.x and 4.0

Severity

annoyance