microsoft word keep seleting the whole text automatically

Is the document perhaps opening in Read Mode? You can tell if the ribbon has been replaced by menus: File, Tools, and View. To exit Read Mode, press the ESC key.

To prevent documents from opening in Read Mode, clear the option "Open e-mail attachments and other uneditable files in reading view" at File > Options > General.

Another possibility is that the document is in protected view, in which case you would see a yellow bar at the top of the document window. Click to enable editing.

Dear MUHAMMAD RAMZAN1 Thank you for your quick response and we also appreciate you spent your precious time doing the tests and providing the detailed information to us.Per your update, it seems that the behavior only happened in Word. To confirm if the issue is related to Word application itself. Please try to create a new user profile then sign in Word to see the behavior.If the behavior still persists, please uninstall your Word app then re-install it from this link  Microsoft Word 2021 - Download (fileplanet.com)Welcome to update the latest situation to us, we'll continue to help you forever!

Sincerely,

Stacey | Microsoft Community Moderator

i've tried extend selection uncheck method, but the issue remains same

also used external keyboard ,

keyboard is working fine in other applications, issue is just with microsoft word.

Dear MUHAMMAD RAMZAN1

Good day! Thank you for posting in Microsoft Community. We are happy to help you.

Based on your description, it seems that Word always selects the whole test automatically on your end. 

First make sure that the document opens in Print Layout view. On the Viewtab, click Print Layout. Please try to right-click the status bar at the bottom of the Word and check the "Selection Mode" option, then check if the "Extended Selection" mode is turned on, if so, you can press ESC to turn it off.

Besides, if you have any other keyboard devices, you can try to use another one to see if the problem will persist.

If the above suggestions cannot help, please provide the following information to help us to better narrow down the cause:

1.    Check the performance in safe mode: Open Office apps in safe mode on a Windows PC

2.    Check if the problem occurs in other apps, such as Excel, PowerPoint, Notepad, etc.

3.    Go to Files > Account> take a screenshot of the Product Information, as shown below, please note to mask your email address.

We’re looking forward to your reply and will continue to help you all the time!  If there are any misunderstanding or unclear, you can post back in your free time.

Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!

Sincerely,

Stacey | Microsoft Community Moderator