Tab not filling the whole screen

Hi GL. I'm Greg, an installation specialist and 9 year Windows MVP, here to help you.

A tab should not be any different than a normal Window since it always resides in one even if you drag it off the present Window.

Easiest way to resize any Window is to click the middle box in the top right corner (between the - and red X) until you can resize the Window. Drag it to the size you want, then click the middle box again a few times until it programs it's size. Do this even if you want it full size.

Other ways to resize any Window:
https://www.computerhope.com/issues/ch001478.htm
https://www.raymond.cc/blog/how-to-resize-an-un...

I hope this helps. Feel free to ask back any questions and keep me posted. If you will wait to choose if I resolved your problem, I will keep working with you until it's resolved.

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thank you Greg, but I've tried to use the maximise square at the top right of the tab, but it appears greyed out, and clicking it doesn't do anything. I've tried dragging the tab to the top of the screen where most tabs would maximise automatically, but that does nothing too. do you know if there is there another solution?

Yes, I gave you all other solutions in the other tutorials, did you read them?

Try another browser. The days of having only one browser are long gone. I would have several browsers on your task bar so there's always one that's best for the task. I use Firefox, Edge and Chrome in that order of preference. It's easy to share bookmarks between all of them in their settings under Import Favorites from another Browser, and Firefox and Chrome let you sign in to sync your Favorites on any other device you sign into. Set the preferred browser as Default in Settings > Apps > Default Apps.

I've never heard of the browser maximize button being greyed out. Is it greyed out on all tabs including the start tab? What browser is this? I'd reset or reinstall it until it works correctly:
https://www.howtogeek.com/171924/how-to-reset-y...

Make sure you back up your Favorites or sign into Firefox and Chrome sync to save Favorites, passwords and history before resetting or reinstalling.

thanks greg this helped and now its fixed, i manually reseted the browser instead of the computer. many thanks, GL

Really glad to hear it, GL. Let me know if there's anything else.

Also when ready please mark the post which helped most as the Answer, to help others. A rating is also appreciated.