The modern Test Case Editor (gui/templates/testcases/tcEdit.html + api/testcasesedit/index.php) regains the legacy design-time custom fields the 1.9.20 editor rendered in the design view.

Legacy testcaseEdit.php?doAction=edit renders ONE OR MORE custom-field blocks (gui/templates/dashio/testcases/tcEditViewer.inc.tpl:47-137), each covering a subset of the CFs linked to the test project, positioned at the 7 legacy design-time locations; values live in cfield_design_values and are persisted on save by testcaseCommands::show() after update() via cfield_mgr->design_values_to_db() (lib/testcases/testcaseCommands.class.php:1228-1235).

The modern editor shipped no custom-field rendering and no persistence — the edit payload carried neither per-location HTML nor CF metadata, so all design-time CFs silently disappeared and their saved values stayed frozen.

BFF api/testcasesedit/index.php:

• tceDesignCustomFields() — renders per-location legacy input HTML via testcase::html_table_of_custom_field_inputs($tcaseId, null, 'design', '', $tcverId, null, $tprojId, $locFilter) for every entry of buildCFLocationMap() (skip_location is left true, so hidden hide_because_is_used_as_variable CFs are not rendered), plus a structured meta list from get_linked_cfields_at_design (id / name / label / type / type_verbose / possible_values / required / location / current value from cfield_design_values).
• edit payload gains custom_fields: {locations, html, meta}.
• update handler: when body.custom_fields is a hash shaped exactly like the legacy $_REQUEST (custom_field_<type>_<id> …), rebuilds the project design-CF map via cfield_mgr->getLinkedCfieldsAtDesign(['tproject_id'=>..,'enabled'=>1,'node_type'=>'testcase'], ['location'=>[1..7]]) and persists through cfield_mgr->design_values_to_db($body['custom_fields'], $tcverId, $cfMap). The location filter keeps hidden variable-CFs (location 8) whose values are never submitted by the editor from being wiped by _build_cfield's seed-and-overwrite logic.

HTML gui/templates/testcases/tcEdit.html:

• 7 location blocks: cf_after_title, cf_before_summary, cf_after_summary, cf_before_preconditions, cf_after_preconditions, cf_before_steps_results, cf_standard_location, styled with .design-cf-block Dashio CSS.
• renderCustomFields() injects each served block verbatim inside an <form onsubmit="return false" novalidate> wrapper — the legacy markup (and its this.form.<name> references used by the text-area counter) works without a real <form>, and novalidate prevents native HTML5 constraint bubbles from ever intercepting save.
• window.textCounter shim (mirror of gui/javascript/validate.js:235) and window.showCal shim (native date-picker overlay writing back through the legacy strftime format) so the served legacy date/textarea markup is fully interactive.
• collectCustomFields() walks the injected block controls and produces the legacy hash (checkbox/radio groups → [] arrays, empty groups → '' so the stored row is deleted); validateCustomFields() runs the required gate plus numeric/float/email checks (mirror of execSetResults.html); both are wired into doSave (custom_fields in the update payload).
• #formCard change-tracking selector extended to checkbox/radio/email/number/date.

i18n: tcedit.cfRequired (The custom field "{field}" is required.) and tcedit.cfInvalid added to ALL 10 locale bundles.

• Browser-verified on fixture tmp/fixtures_1312.php (TCEDDemo, tcA v2): Severity (list) + Priority Flag (checkbox) render after preconditions using the exact legacy markup; Notes (textarea) renders at standard location.
• UI save round-trips all three values through cfield_design_values; reload re-renders the saved state.
• Required gate measured: empty required field blocks save with The custom field "Severity" is required.
• Browser console clean (only pre-existing a11y "no label" issues), Event Viewer shows only INFO audit rows for the feature saves.
• Suite 1312 7/7 PASS in tmp/TLU_Test_Cases.md.

• Bug #1571: design-time datetime CF values are wiped on save because legacy cfield_mgr::_build_cfield (lib/functions/cfield_mgr.class.php:1924-1973) reads $value['input'] from a last-suffix-wins array → E_WARNING + DELETE. Filed separately (repro + measured evidence); fix belongs in the legacy class, outside this task's scope.