Modernization of the legacy compare-versions screen (lib/testcases/tcCompareVersions.php, gui/templates/dashio/testcases/tcCompareVersions.tpl) — GitHub issue #809.

The legacy Smarty screen is replaced by a standalone Dashio page (gui/templates/testcases/tcCompare.html) backed by a plain-PHP REST BFF API (api/testcasescompare/index.php). The Test Case Viewer toolbar Compare Versions button now opens the modern screen instead of submitting the legacy POST form.

Path: Test Spec → Test Case Viewer → toolbar Compare Versions URL: gui/templates/testcases/tcCompare.html?tproject_id=..&testcase_id=.. BFF API: GET /api/testcasescompare/?action=info + GET /api/testcasescompare/?action=compare Rights: any authenticated session (parity with legacy, which is read-only). Tracking issue: #809

---

Section	Description
Heading	Compare title + test-case context (name + id)
Version table	One row per version: version number, radio Version A (left), radio Version B (right), modified/created timestamp, author
Diff method	Radio HTML (code) comparison (DaisyDiff) vs Text (inline) comparison (legacy line diff)
Context rows	Only for text mode: Context lines number + Show all toggle
Action	Compare selected versions and Cancel

Default selection mirrors legacy: the newest version is pre-selected on the right and the previous one on the left (matching the legacy {if $mycount == 2} / {if $mycount == 1} defaults).

1. On load the page calls GET /api/testcasescompare/?action=info to resolve the test case, its version list (with modification/creation timestamp + author) and the default text-diff context.
2. The user picks two different versions and a diff method, then clicks Compare selected versions.
3. The page calls GET /api/testcasescompare/?action=compare with version_left, version_right, use_html_comp, and (text mode) context / context_show_all.
4. The BFF runs the same third_party diff engines as the legacy controller (third_party/diff/diff.php inline() for text, HTMLDiffer for HTML) and returns per-section diffs. The client renders each section with an i18n heading and a localized change-count message.

Key	Source
summary	tcversions.summary (left vs right version)
preconditions	tcversions.preconditions
steps	concatenated step actions with line breaks
expected_results	concatenated step expected results with line breaks

Case	Result
No testcase_id	warn box tcc.errNoTestcase, Compare disabled
Test case not found	HTTP 404 {"status":"error","message":"Test case not found"}
Same version selected twice	local toast tcc.warnSameVersions, no request
Blank / non-numeric / negative context	local toast tcc.warnInvalidContext, no request
Explicit context 0	sent to the BFF as context=0 and honored end-to-end (diff renders only the changed lines, see below)
Not authenticated	HTTP 401 {"status":"error","message":"Not authenticated"}

Gap-fix restoring legacy parity for the Text (inline) diff context field (lib/testcases/tcCompareVersions.php:97-106 init_args + gui/templates/dashio/testcases/tcCompareVersions.tpl:77-91 validateForm):

• Legacy treats a numeric context value exactly as given — 0 is honored (the diff engine's showline() has an explicit linepadding === 0 branch that prints only changed lines, third_party/diff/diff.php:79). It only falls back to the diffEngine config default (config_get('diffEngine')->context, typically 5) when the parameter is absent or non-numeric.
• Pre-fix regressions (both from the original #809 port): a blank field was silently coerced to 0 by doCompare, and an explicit 0 was dropped to the config default by the BFF — both diverging from legacy.
• Fix: tcCompare.html doCompare() now reads the raw field value; blank/null, NaN or context < 0 trigger the tcc.warnInvalidContext toast and nothing is sent to the BFF. api/testcasescompare/index.php parses the parameter with an explicit -1 sentinel ($context = ($rawCtx >= 0) ? $rawCtx : $defaultCtx; where $defaultCtx = diffEngine->context || 5); context_show_all=1 still maps to context = null (render everything).
• i18n: the tcc.warnInvalidContext key already existed in all 10 bundles — no new keys were required for this fix.

• The BFF reuses the exact legacy diff logic (buildDiff() in tcCompareVersions.php), including the step concatenation with <br /><br /> separators and the </p> → </p>\n line-break insertion for the text engine.
• Event Viewer fix: the legacy controller resolved the section messages and subtitle with lang_get('num_changes'/'no_changes'/'diff_subtitle_tc'). Those strings are missing from most locale/*/strings.txt, which fired E_WARNING Undefined array key into the Event Viewer on every compare. The BFF now returns raw counts + version ids and the client resolves all messages via TLi18n keys (tcc.numChanges, tcc.noChanges, tcc.subtitle, tcc.heading.*), so the Event Viewer stays clean.

Method	Endpoint	Description
GET	/api/testcasescompare/?action=info	test case name + version list + default context
GET	/api/testcasescompare/?action=compare	side-by-side / inline diff of two versions

All routes require a valid session (return 401 otherwise).

All labels, titles, headings, messages and placeholders use client-side TLi18n keys under the tcc.* namespace (30 keys) present in all 10 locale bundles (gui/templates/i18n/{en,ro,de,es,fr,it,ja,pt,ru,zh}.json). Bundle consistency is enforced by tools/lint_i18n.py; every bundle passes python3 -m json.tool.

---

TestLink 2.0.1 · Compare Test Case Versions · Refs #809, #1327