Add dynamic captcha support

inc/config.php

@@ -351,6 +351,15 @@
 	//);
 	$config['simple_spam'] = false;
 
+	/*
+	 * If not flase, the captcha is dynamically injected on the client if the web server set the `captcha-required`
+	 * cookie to 1. The configuration value should be set the IP for which the captcha should be verified.
+	 *
+	 * Example:
+	 * $config['dynamic_captcha'] = '127.0.0.1'; // Verify the captcha for users sending posts from the loopback address.
+	 */
+	$config['dynamic_captcha'] = false;
+
 	// Enable reCaptcha to make spam even harder. Rarely necessary.
 	$config['recaptcha'] = false;
 

post.php

@@ -648,7 +648,7 @@ if (isset($_POST['delete'])) {
 				}
 			}
 			// Remote 3rd party captchas.
-			else {
+			else if (!$config['dynamic_captcha'] || $config['dynamic_captcha'] === $_SERVER['REMOTE_ADDR']) {
 				// recaptcha
 				if ($config['recaptcha']) {
 					if (!isset($_POST['g-recaptcha-response'])) {

templates/main.js

@@ -222,6 +222,36 @@ function getCookie(cookie_name) {
 	}
 }
 
+{% endraw %}
+{% if config.dynamic_captcha %}
+function is_dynamic_captcha_enabled() {
+	let cookie = get_cookie('require-captcha');
+	return cookie === '1';
+}
+
+function get_captcha_pub_key() {
+{% if config.recaptcha %}
+	return "{{ config.recaptcha_public }}";
+{% else %}
+	return null;
+{% endif %}
+}
+
+function init_dynamic_captcha() {
+	if (!is_dynamic_captcha_enabled()) {
+		let pub_key = get_captcha_pub_key();
+		if (!pub_key) {
+			console.error("Missing public captcha key!");
+			return;
+		}
+
+		let captcha_hook = document.getElementById('captcha');
+		captcha_hook.style = "";
+	}
+}
+{% endif %}
+{% raw %}
+
 function highlightReply(id) {
 	if (typeof window.event != "undefined" && event.which == 2) {
 		// don't highlight on middle click

templates/post_form.html

@@ -73,7 +73,11 @@
 			</td>
 		</tr>
 		{% if config.recaptcha %}
+		{% if config.dynamic_captcha %}
+		<tr id="captcha" style="display: none;">
+		{% else %}
 		<tr>
+		{% endif %}
 			<th>
 				{% trans %}Verification{% endtrans %}
 				{{ antibot.html() }}