Selecting an existing published page (e.g. the homepage) as the maintenance page could permanently break it (#523): the plugin forced it private on every load with maintenance mode disabled, and applying a gallery template overwrote its content. The plugin now records a selected page's original state and restores it, and only writes templates into pages it created itself.

What changed

• select_page() (or first enable, for pre-existing selections) records the page's original status and template in a single post meta value, claimed first-write-wins, so a racing request can neither capture plugin-made changes as "original" nor observe half a record. The record is cleared after each restore, so later user edits become the new baseline; a maintenance template carried over from an old release is recorded as "no template" rather than as user state.
• Disabling maintenance mode used to force the page private; disabling and plugin deactivation now restore the recorded status and template, and leave pages with no record untouched.
• Restore only reverts a status the plugin itself applied (its publish of a private page). A status the user chose while maintenance mode was active — draft, private, or trash — is kept, and the page just gets its template handed back and the record cleared. Enable likewise skips deleted and trashed pages entirely.
• Changing or clearing the page selection restores the previously selected page before switching, so an abandoned page doesn't keep the maintenance template or a stale record.
• Template imports only overwrite pages the plugin created (marked _wpmm_generated); a selected user-owned page is restored and the template lands on a newly created "Maintenance Page", which becomes the selection. The "content will be replaced" warning in settings now shows only in that overwrite case.
• Template imports reject slugs/categories that don't resolve to a bundled template (previously created an empty page; both are sanitized before filesystem use), and the Otter CSS_Handler call is skipped when Otter Blocks isn't installed (previously a fatal error).

The CI job also moves from PHP 7.2 to 7.4: the latest WordPress test suite requires 7.4, so PHPUnit aborted before running any test. It's bundled here because this PR's tests depend on it.

Lifecycle after this PR

flowchart TD
    A[User selects a page] --> P[Restore previously<br/>selected page, if any]:::added
    P --> B[Record original status + template]:::added
    B --> C[Enable: apply wpmm template,<br/>publish page, mark publish as plugin-made]:::changed
    C --> D{Disable / deactivate}
    D --> E[Undo the plugin-made publish only,<br/>restore template, clear record;<br/>user status changes stay]:::added

    F[Template import] --> G{Page marked<br/>_wpmm_generated?}:::added
    G -- yes --> H[Overwrite it with the template]
    G -- no --> I[Restore user page; create new<br/>generated page and select it]:::added

    classDef added fill:#1a7f37,color:#fff,stroke:#1a7f37
    classDef changed fill:#9a6700,color:#fff,stroke:#9a6700

🟩 added by this PR (restore on switch, record/restore, ownership check, new-page branch) · 🟨 changed behavior (enable now records state, marks its publish, and skips deleted/trashed pages) · gray = unchanged

Data changes

Before: no stored record — disable ran post_status → private unconditionally. After: state is recorded on takeover and consumed on restore.

QA

1. Regression (Disabling maintenance mode makes the selected homepage private and returns 404 #523): select an existing published page as the maintenance page, enable maintenance mode, then disable it. The page stays public with its original template — previously it went private and visitors got a 404.
2. With maintenance mode active and your page selected, switch the page to Draft in the editor, then disable maintenance mode. The page stays a draft instead of being republished.
3. With page A selected, pick a different page B (or clear the selection) in Design → Select page. Page A returns to its original status and template; B becomes the maintenance page.
4. Select a page, trash it, then deactivate the plugin and restore the page from the trash. It comes back without the maintenance template.
5. With a user-owned page selected, open Design → Templates: no "content will be replaced" warning. Select a plugin-generated page: the warning appears.
6. On a site without Otter Blocks, apply a gallery template: it applies without a fatal error.