feat(core polyfills): Add polyfills module for modern browsers.

Currently this holds a polyfill for SubmitEvent.submitter, which isn't available in Safari < 15.4 and jsDOM.

Author: thet

src/core/polyfills.js

@@ -0,0 +1,35 @@
+// Polyfills for modern browsers
+
+// SubmitEvent.submitter polyfill for Safari < 15.4 and jsDOM
+// Original code: https://stackoverflow.com/a/61110260/1337474
+// Also see: https://caniuse.com/?search=submitter
+//
+!(function () {
+    let last_btn = null;
+    const submitable_buttons = `button, input[type="button"], input[type="submit"], input[type="image"]`;
+    document.addEventListener(
+        "click",
+        function (e) {
+            if (!e.target.closest) return;
+            last_btn = e.target.closest(submitable_buttons);
+        },
+        true
+    );
+    document.addEventListener(
+        "submit",
+        function (e) {
+            if ("submitter" in e) return;
+            const canditates = [document.activeElement, last_btn];
+            last_btn = null;
+            for (const candidate of canditates) {
+                if (!candidate) continue;
+                if (!candidate.form) continue;
+                if (!candidate.matches(submitable_buttons)) continue;
+                e.submitter = candidate;
+                return;
+            }
+            e.submitter = e.target.querySelector(submitable_buttons);
+        },
+        true
+    );
+})();