Bug: Outbound link tracking broken with REST transport + sendBeacon

Summary

External link (outbound) tracking, download tracking, and exit-time updates are broken when using the default REST API transport. The JavaScript tracker sends interaction data via navigator.sendBeacon() (Content-Type: text/plain), but the REST handler overwrites correctly-parsed tracking data with an empty/incomplete payload.

Root Cause

TrackingRestController::handle_tracking() (line 131-145) calls $request->get_params() and overwrites wp_slimstat::$raw_post_array. WordPress REST API does not parse text/plain request bodies, so get_params() returns empty data for sendBeacon requests — destroying the correct data already parsed from php://input at plugin init (wp-slimstat.php:1559-1575).

Introduced by: commit cfb09a21 (2026-02-21, "fix not collecting data")

Impact

Reproduction

1. Install WP SlimStat with default settings (REST transport, javascript_mode=off)
2. Visit any page - pageview tracked correctly
3. Click an external link (e.g., https://www.google.com)
4. Check access log - outbound_resource is empty

Suggested Fix

In handle_tracking(), merge with the already-parsed raw_post_array instead of overwriting:

$payload = $request->get_params();
if (!is_array($payload)) {
    $payload = [];
}
// Preserve sendBeacon text/plain data already parsed from php://input
if (!empty(\wp_slimstat::$raw_post_array)) {
    $payload = array_merge($payload, \wp_slimstat::$raw_post_array);
}
$payload['action'] = 'slimtrack';
\wp_slimstat::$raw_post_array = $payload;

Code References

Environment

• WP SlimStat (current main branch)
• Default REST transport
• Any modern browser with sendBeacon support