Error: must implement interface TYPO3\CMS\Backend\View\PageLayoutViewDrawItemHookInterface about inline_page_editing

[NamelessCoder]

Needs composer install (this error is raised when the ContentPreview class from this extension cannot be loaded by the composer autoloader) - if you clone the repository into typo3conf/ext manually then you must add class loading information manually, for example in the root composer.json of the site:

{
    "require": {
        "typo3/cms": "^7.6",
        "namelesscoder/inline-page-editing": "^1.0"
    },
    "autoload": {
        "psr-4": {
            "NamelessCoder\\InlinePageEditing\\": "typo3conf/ext/inline_page_editing/Classes/"
        }
    }
}

(and then composer dump-autoload).

You are correct about the version. Forgot to increment it in ext_emconf.php - will fix.

Didn't do it manually; did a composer require namelesscoder/inline-page-editing and then activated extension in TYPO3 via Extension Manager. I have the require line in composer.json, but the autoload line wasn't added... why not?

[NamelessCoder]

I have only been able to reproduce this one on sites where the files were manually uploaded and composer install or composer dump-autoload was not executed. I have checked this on a case insensitive file system as well, so it doesn't seem to be the class loader either. Are you sure this one is an issue? Could there be external causes why the class loading might fail (symlink structure, location of composer relative to project, changed vendor-dir, other...?)

I have the require line in composer.json, but the autoload line wasn't added... why not?

It is only needed when you install the extension manually and it will never be added automatically, so that's as expected.

[NamelessCoder]

Could not reproduce and no other users report the problem - taking the liberty of closing this and assuming it was caused by class loader caches or other installation-related issues.