Hello,
after enabling and testing the REST API in MantisBT 2.28, I found an issue related to page rendering mode.
Environment:
- MantisBT version: 2.28
- Web server: nginx + PHP-FPM
- Browser: Chrome
- REST API enabled with:
$g_webservice_rest_enabled = ON;
Problem:
The REST API initially failed with:
RuntimeException: Unexpected data in output buffer. Maybe you have characters before an opening <?php tag?
in /vendor/slim/slim/Slim/App.php
After investigation, I found that config/custom_strings_inc.php contained a UTF-8 BOM at the beginning of the file.
With BOM present:
- REST API fails with the Slim output buffer error.
- Main UI visually renders correctly.
- document.compatMode in Chrome returns:
BackCompat
- The HTML response starts with several BOM bytes before <!DOCTYPE html>:
ef bb bf ef bb bf ... 3c 21 44 4f 43
After removing BOM:
- REST API works correctly.
- HTML starts correctly with:
<!DOCTYPE html>
- document.compatMode returns:
CSS1Compat
- However, the MantisBT UI becomes visually broken, as if styles/layout are not applied correctly.
I tested the following:
- Removed BOM from custom_strings_inc.php.
- Verified that the response starts with <!DOCTYPE html>.
- Verified that REST API works after removing BOM.
- Disabled plugins.
- Disabled custom_strings_inc.php.
- Commented custom status configuration.
- CSS files are loaded with HTTP 200.
- css/status_config.php returns text/css and valid CSS.
- PHP syntax check for custom_strings_inc.php reports no errors.
- The issue still occurs in CSS1Compat mode.
- As a temporary workaround, removing/commenting the <!DOCTYPE html> output in core/html_api.php forces BackCompat mode and the UI renders correctly while REST API continues to work.
Expected result:
MantisBT UI should render correctly with:
- no UTF-8 BOM in PHP files,
- <!DOCTYPE html> present as the first output,
- document.compatMode = CSS1Compat,
- REST API working normally.
Actual result:
The UI appears broken in CSS1Compat mode, but renders correctly in BackCompat mode.
Temporary workaround:
Remove/comment <!DOCTYPE html> output in core/html_api.php to force BackCompat mode. This avoids using BOM and does not break REST API, but it is not a proper fix.
Question:
Is this a known issue in MantisBT 2.28, or is there any configuration/CSS setting required to make the UI render correctly in CSS1Compat mode? | 
