Upstreaming from https://phabricator.wikimedia.org/T284397
1. Phorge at `8d2b481bb5a83d1a4ce82014cabdc83fa3ca432d`, PHP 8.2.5 (though irrelevant here)
2. Go to an existing task.
3. Write the comment `a {{{a}}} a`
```
[2023-05-09 12:12:57] EXCEPTION: (RuntimeException) Undefined array key 1 at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=renderDashboardTabPanelContent, ref.master=d47289622650, ref.renderDashboardTabPanelContent=d47289622650), phorge(head=master, ref.master=8d2b481bb5a8)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/infrastructure/markup/remarkup/PhutilRemarkupEngine.php:243]
#1 <#2> PhutilRemarkupEngine::splitTextIntoBlocks(array) called at [<phorge>/src/infrastructure/markup/remarkup/PhutilRemarkupEngine.php:136]
#2 <#2> PhutilRemarkupEngine::preprocessText(string) called at [<phorge>/src/infrastructure/markup/remarkup/PhutilRemarkupEngine.php:106]
#3 <#2> PhutilRemarkupEngine::markupText(string) called at [<phorge>/src/infrastructure/markup/PhabricatorMarkupEngine.php:614]
#4 <#2> PhabricatorMarkupEngine::extractPHIDsFromMentions(PhabricatorUser, array) called at [<phorge>/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php:2059]
#5 <#2> PhabricatorApplicationTransactionEditor::buildSubscribeTransaction(ManiphestTask, array, array) called at [<phorge>/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php:2221]
#6 <#2> PhabricatorApplicationTransactionEditor::expandSupportTransactions(ManiphestTask, array) called at [<phorge>/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php:1222]
#7 <#2> PhabricatorApplicationTransactionEditor::applyTransactions(ManiphestTask, array) called at [<phorge>/src/applications/transactions/editengine/PhabricatorEditEngine.php:2031]
#8 <#2> PhabricatorEditEngine::buildCommentResponse(ManiphestTask) called at [<phorge>/src/applications/transactions/editengine/PhabricatorEditEngine.php:998]
#9 <#2> PhabricatorEditEngine::buildResponse() called at [<phorge>/src/applications/maniphest/controller/ManiphestTaskEditController.php:12]
#10 <#2> ManiphestTaskEditController::handleRequest(AphrontRequest) called at [<phorge>/src/aphront/configuration/AphrontApplicationConfiguration.php:284]
#11 phlog(RuntimeException) called at [<phorge>/src/aphront/handler/PhabricatorAjaxRequestExceptionHandler.php:27]
#12 PhabricatorAjaxRequestExceptionHandler::handleRequestThrowable(AphrontRequest, RuntimeException) called at [<phorge>/src/aphront/configuration/AphrontApplicationConfiguration.php:751]
#13 AphrontApplicationConfiguration::handleThrowable(RuntimeException) called at [<phorge>/src/aphront/configuration/AphrontApplicationConfiguration.php:296]
#14 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink, MultimeterControl) called at [<phorge>/src/aphront/configuration/AphrontApplicationConfiguration.php:203]
#15 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phorge>/webroot/index.php:35]
```
Desired situation (without any crash):
{F293294, size=full}