The modern Test Case Editor (gui/templates/testcases/tcEdit.html) regains the legacy Assign Requirements affordance that was dropped during modernization: nothing on the editor let a user link requirements to the test case being edited.

{if $gui->opt_requirements==TRUE && $gui->grants->req_tcase_link_management=='yes' && isset($gui->tc.testcase_id)}
  <br /><div><a href="javascript:openReqWindow({$gui->tc.testcase_id})">{$labels.assign_requirements}</a></div>
{/if}

• opt_requirements — project option requirementsEnabled (tcEdit.php:406-412);
• req_tcase_link_management — right 28 (tcEdit.php:654-662);
• the link opens a popup lib/requirements/reqTcAssign.php?id=<tcase_id>&edit=testcase (testcase mode) that lists Assigned + Free requirements per requirement spec and assigns/unassigns via POST (assign_to_tcase, delReqVersionTCVersionLinkByID); the whole popup feature is gated on right 28 (reqTcAssign.php:402-407).

• BFF api/testcasesedit/index.php buildEditPayload():
  • reads req_tcase_link_management right via $user->hasRight($db, 'req_tcase_link_management', $tprojId);
  • reads requirementsEnabled from the project options blob (intval($projRow['opt']->requirementsEnabled ?? 0)), same source as legacy;
  • emits grants.req_tcase_link_management and top-level requirements_enabled.
• HTML gui/templates/testcases/tcEdit.html:
  • toolbar button #btnAssignReqs (fa-bookmark) shown only when ctx.info.requirements_enabled && ctx.info.grants.req_tcase_link_management (exact legacy gate minus the always-true-in-editor isset(testcase_id) check);
  • #assignReqsModal — same Free/Assigned dual-select layout as the already-modernized tcView.html:129-162 / testSpec.html;
  • arqApiGet/arqApiPost/arqOptLabel/openAssignReqs/arqLoadReqs/arqDoAssign/ arqDoUnassign — same client logic as tcView.html:547-646, hitting the shared /api/requirements router: GET /assign-reqspecs, GET /assign-reqs?req_spec_id=&tcase_id=, POST /assign-reqs {tcase_id, req_ids[]}, POST /unassign-reqs {link_ids[]}. Those endpoints already enforce right 28 (arNeedManageRight), so the client gate is a UX nicety and the security boundary is unchanged.
• i18n — tcedit.assignRequirements added to all 10 locale bundles (values mirroring the existing tcview.assignRequirements translations); the modal reuses reqAssign.* + tspec.cancel + rpt.close (already present).

• requirements_enabled=1 + right present → button visible; flipping the project option to 0 hides it (offsetParent === null).
• Modal lists [TCED-SPEC1] - TCED-SPEC1, Free contains TCED-REQ1 - Req A (v. 1).
• Assign → toast "1 requirement(s) assigned.", row moves to Assigned, req_coverage row created (req_id 3, req_version_id 4, testcase_id 5, tcversion_id 6, link_status 1, author 1), INFO audit event audit_reqv_assigned_tcv.
• Unassign → toast "1 link(s) removed.", row deleted, INFO audit event audit_reqv_assignment_removed_tcv.
• Save-flow regression green ("Test case saved.", summary persisted).
• Event Viewer: only log_level-16 audits, zero Error/Warning. Browser console clean.

Screenshot: docs/screenshots/issue-1313-tcedit-assign-requirements-modal.png.