[fix] no false positive warnings for fetch uses in firefox (#8456)

fixes #7992

.changeset/shy-readers-help.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+[fix] no false positive warnings for fetch uses in firefox

packages/kit/src/runtime/client/fetcher.js

@@ -24,12 +24,20 @@ if (DEV) {
 	check_stack_trace();
 
 	window.fetch = (input, init) => {
+		// Check if fetch was called via load_node. the lock method only checks if it was called at the
+		// same time, but not necessarily if it was called from `load`.
+		// We use just the filename as the method name sometimes does not appear on the CI.
 		const url = input instanceof Request ? input.url : input.toString();
-		const stack = /** @type {string} */ (new Error().stack);
+		const stack_array = /** @type {string} */ (new Error().stack).split('\n');
+		// We need to do some Firefox-specific cutoff because it (impressively) maintains the stack
+		// across events and for example traces a `fetch` call triggered from a button back
+		// to the creation of the event listener and the element creation itself,
+		// where at some point client.js will show up, leading to false positives.
+		const firefox_cutoff = stack_array.findIndex((a) => a.includes('*listen@'));
+		const stack = stack_array
+			.slice(0, firefox_cutoff !== -1 ? firefox_cutoff : undefined)
+			.join('\n');
 
-		// check if fetch was called via load_node. the lock method only checks if it was called at the
-		// same time, but not necessarily if it was called from `load`
-		// we use just the filename as the method name sometimes does not appear on the CI
 		const heuristic = can_inspect_stack_trace
 			? stack.includes('src/runtime/client/client.js')
 			: loading;