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How to remove 255 characters limit from meta keywords field in product in admin while saving or updating existing ?

After some debugging I found out that all inputs have 255 length limit and they all have the same template.
The limit is set by the html template itself (kinda hard-coded).
The file path is:

vendor/magento/module-ui/view/base/web/templates/form/element/input.html

So we have to override it into a custom adminhtml theme.

Before you start, please replace the "Vendor" and "Theme" with your "Vendor" and "Module" names

If you do not have one:

Create the following files:

app/design/adminhtml/Vendor/Theme/composer.json

"name": "<vendor>/<theme>",
"description": "N/A",
"require": 
 "php": "~5.5.0,
"type": "magento2-theme",
"version": "1.0.0",
"license": [

],
"autoload": 
 "files": [
 "registration.php"
 ]

app/design/adminhtml/Vendor/Theme/registration.php

<?php

MagentoFrameworkComponentComponentRegistrar::register(
 MagentoFrameworkComponentComponentRegistrar::THEME,
 'adminhtml/<Vendor>/<Theme>',
 __DIR__
);

app/design/adminhtml/Vendor/Theme/theme.xml

<theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
 <title>Theme</title>
 <parent>Magento/backend</parent>
</theme>

These are the theme files.

Now we have to create a custom module to make this work:

app/code/Vendor/Theme/registration.php

<?php

MagentoFrameworkComponentComponentRegistrar::register(
 MagentoFrameworkComponentComponentRegistrar::MODULE,
 'Vendor_Theme',
 __DIR__
);

app/code/Vendor/Theme/composer.json

 "name": "vendor/theme",
 "description": "",
 "require": 
 "php": "~5.5.0,
 "suggest": 

 ,
 "type": "magento2-module",
 "version": "0.1.0",
 "license": [

 ],
 "autoload": 
 "files": [
 "registration.php"
 ],
 "psr-4": 
 "Vendor\Theme\": ""
 
 ,
 "extra": 
 "map": [
 [
 "*",
 "Vendor/Theme"
 ]
 ]
 

app/code/Vendor/Theme/etc/module.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
 <module name="Vendor_Theme" setup_version="0.1.0">
 </module>
</config>

app/code/Vendor/Theme/etc/di.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
 <type name="MagentoThemeModelViewDesign">
 <arguments>
 <argument name="themes" xsi:type="array">
 <item name="adminhtml" xsi:type="string">Vendor/Theme</item>
 </argument>
 </arguments>
 </type>
</config>

Now we have a custom adminhtml(backend) theme that overrides the Magento_Backend theme.

Here comes the magic:

The input template file for ui components is:

vendor/magento/module-ui/view/base/web/templates/form/element/input.html

We copy it in a relative path in our custom theme:

app/design/adminhtml/Vendor/Theme/Magento_Catalog/web/form/element/input.html

We modify the content into:

<!--Overrides: vendor/magento/module-ui/view/base/web/templates/form/element/input.html -->
<!-- ko if: inputName == "product[meta_keyword]" -->
<input class="admin__control-text" type="text"
 data-bind="
 event: change: userChanges,
 value: value,
 hasFocus: focused,
 valueUpdate: valueUpdate,
 attr: 
 name: inputName,
 placeholder: placeholder,
 'aria-describedby': noticeId,
 id: uid,
 disabled: disabled
 "/>
<!--/ko-->
<!-- ko ifnot: inputName == "product[meta_keyword]" -->
<input class="admin__control-text" type="text"
 data-bind="
 event: change: userChanges,
 value: value,
 hasFocus: focused,
 valueUpdate: valueUpdate,
 attr: 
 name: inputName,
 placeholder: placeholder,
 'aria-describedby': noticeId,
 id: uid,
 disabled: disabled,
 maxlength: 255
 "/>
<!-- /ko -->

As you can see here, we tell to knockoutjs to render the input without the maxlength condition only when the input name is "product[meta_keyword]" (thats our input we want)
else, we render the input normally to not do something we don't want to.

Now, we have to make our template override the Magento_Ui template file:

As we see in the Magento_Ui requirejs_config.js of base,

vendor/magento/module-ui/view/base/requirejs-config.js

There is a namespace for the template paths of the ui components:

....
paths: 
 'ui/template': 'Magento_Ui/templates'
,
....

That means all the templates paths that are from "Magento_Ui/templates" are replaced with "ui/template".

So, we create our requirejs_config.js:

app/design/adminhtml/Vendor/Theme/Magento_Catalog/requirejs-config.js

var config = 
 map: 
 "*": 
 "ui/template/form/element/input": "Magento_Catalog/form/element/input"
 
 
;

Now clear the cache, clear the static files (or deploy them) and refresh the page, and you should see something like this:

This works for 2.3.1, and it should work for the lower versions