Move internal packages to an internal folder

For reference: https://go.dev/doc/go1.4#internalpackages

internal/ui/static/js/modal_handler.js

@@ -0,0 +1,101 @@
+class ModalHandler {
+    static exists() {
+        return document.getElementById("modal-container") !== null;
+    }
+
+    static getModalContainer() {
+        return document.getElementById("modal-container");
+    }
+
+    static getFocusableElements() {
+        let container = this.getModalContainer();
+
+        if (container === null) {
+            return null;
+        }
+
+        return container.querySelectorAll('button, [href], input, select, textarea, [tabindex]:not([tabindex="-1"])');
+    }
+
+    static setupFocusTrap() {
+        let focusableElements = this.getFocusableElements();
+
+        if (focusableElements === null) {
+            return;
+        }
+
+        let firstFocusableElement = focusableElements[0];
+        let lastFocusableElement = focusableElements[focusableElements.length - 1];
+
+        this.getModalContainer().onkeydown = (e) => {
+            if (e.key !== 'Tab') {
+                return;
+            }
+
+            // If there is only one focusable element in the dialog we always want to focus that one with the tab key.
+            // This handles the special case of having just one focusable element in a dialog where keyboard focus is placed on an element that is not in the tab order.
+            if (focusableElements.length === 1) {
+                firstFocusableElement.focus();
+                e.preventDefault();
+                return;
+            }
+
+            if (e.shiftKey && document.activeElement === firstFocusableElement) {
+                lastFocusableElement.focus();
+                e.preventDefault();
+            } else if (!e.shiftKey && document.activeElement === lastFocusableElement) {
+                firstFocusableElement.focus();
+                e.preventDefault();
+            }
+        };
+    }
+
+    static open(fragment, initialFocusElementId) {
+        if (ModalHandler.exists()) {
+            return;
+        }
+
+        this.activeElement = document.activeElement;
+
+        let container = document.createElement("div");
+        container.id = "modal-container";
+        container.setAttribute("role", "dialog");
+        container.appendChild(document.importNode(fragment, true));
+        document.body.appendChild(container);
+
+        let closeButton = document.querySelector("button.btn-close-modal");
+        if (closeButton !== null) {
+            closeButton.onclick = (event) => {
+                event.preventDefault();
+                ModalHandler.close();
+            };
+        }
+
+        let initialFocusElement;
+        if (initialFocusElementId !== undefined) {
+            initialFocusElement = document.getElementById(initialFocusElementId);
+        } else {
+            let focusableElements = this.getFocusableElements();
+            if (focusableElements !== null) {
+                initialFocusElement = focusableElements[0];
+            }
+        }
+
+        if (initialFocusElement !== undefined) {
+            initialFocusElement.focus();
+        }
+
+        this.setupFocusTrap();
+    }
+
+    static close() {
+        let container = this.getModalContainer();
+        if (container !== null) {
+            container.parentNode.removeChild(container);
+        }
+
+        if (this.activeElement !== undefined && this.activeElement !== null) {
+            this.activeElement.focus();
+        }
+    }
+}