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By default, Steam displays notifications in the bottom right corner of the screen. This is usually not a problem if you are not in the game, but can be problematic if you are in the game, as there may be overlapping in-game items or controls. Notifications don't show up for a long period of time, but it can still be troublesome if you're in a multiplayer game or a single player game that puts you under pressure.

Previously, it was possible to edit the steam.styles file in the Steam folder to change the notification behavior of the Steam client.
Regardless, a recent update appears to have changed this, and while you can still edit the file, your information will be overwritten when you restart Steam. Setting the file to read-only doesn't work either, and while changing security access restrictions might still work, it's not something many users want to do.
How to disable steam pop-up ads
Well, playing a game on Steam is easy but every now and then it creates the problem for the user with its irritating pop-ups. These pop-ups appear even when you basically turn on your laptop. There are different types of popups. I mean they show the ads when we open Steam or while we play. Before turning off the steam popup. Let's start by reading the types of Steam popups.
Ad windows appear on the screen when the laptop is open: These pop-ups appear every time the user opens their computer or when Steam is not open. These popups are mainly used to inform users about discounts and new additions and many other suggestions.
Pop-up notifications throughout the game: These pop-up notifications are displayed while users are playing the game. Friend requests and invites, chats are displayed from these popups.

How to disable pop-up ads
This method doesn't just disable Steam pop-up screens. Ads are also permanently blocked. You can use your Mac or Windows to block popups.
- Open Steam on your computer.
- Sign in to your Steam account now.
- Now click on the Steam option at the top left corner of the home page.
- Select the configuration from the listing.
- Now go to the Interface tab > Notify me.
- You will now see a Notify option at the bottom and a check mark will appear.
https://steamcommunity.com/discussions/forum/1/350542145708135347/
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