Create a new article in Joomla template - give it a reasonable length title:
For example: "Decent Length Title - Search Engine Optimised"
Create a new menu link to that same article
Menu Manager -> Main Menu -> New -> Article -> Single Article Layout
- Enter ONLY the required information.
- Menu Item Title (The text that will appear in menus for this link - keep it short) 'Short menu text'
- On the right click the button to select which article to link to: select your new article
- Specifically - do NOT set the 'Browser Page Title' within 'Page display options'
- Now save and exit - do no more.
- Now on your site navigate to the page in question by clicking on the 'Short menu text' link in the menu.
As expected the heading within the page shows the full article's title.
Now view source - and look for the <title> tag. Do you see that?