There's plenty of ways to overlay a borderless window crosshair. But using borderless windowed mode increases input lag and can adversely affect performance, especially on lower end systems.
DISCLAIMER:
I wanted to make sure that I was not in breach of the EULA before using this method for a custom crosshair overlay, as some methods of overlaying elements in full screen are intrusive and could possibly be used to gain an unfair advantage.
The Blizzard EULA states that you cannot create, use, offer, advertise, make available and/or distribute the following or assist therein:

'any code and/or software, not expressly authorized by Blizzard, that can be used in connection with the Battle.net client, the Service, a Game and/or any component or feature thereof which changes and/or facilitates the gameplay;'

After doing some research I discovered Overwolf, which according to the wiki:

'..has gained traction in competitive video games, such as eSports and MMORPGs, where native extensions are often forbidden due to concerns about cheating. Overwolf extensions sidestep this concern, since they do not interact with the game engine; they operate exclusively on the overlay created by the main Overwolf program.'

Additionally, one of the developers of Overwolf has responded to an Overwatch player's concerns with using the program:

'Hey SuperLoki,
Thank you for reaching out to us mate :)
Thankfully you can be rest assured. Whenever we work on apps or features in Overwolf we are making a great effort to stay in touch with the game-company to ensure we are not in violation of any terms of service or policies.
For argument's sake, we didn't develop anything for Overwatch quite yet, however if your concern is using video recording apps of ours, there are alot of programs out there that do this and are not causing bans of any sort.
Blizzard's main concern right now is people who actually cheat via scripting/aimbots and whatnot in the game itself. Overwolf does not grant any type of unfair advantage of this sort.
I hope this clarifies things up for you, and please feel free to ask me any questions you might have. I'm here for you :)
Have a nice day!'
Link to reddit thread: https://www.reddit.com/r/Overwolf/comments/4rzd6n/overwold_safe_with_overwatch/

Overwatch is also listed as fully supported on Overwolf's official site:
http://www.overwolf.com/supported-games/
From the above information it seems as if the Overwolf developers would assume responsibility if any functionality of their program was exploited in a way that could harm the game experience/community in Overwatch, and would attempt to work with Blizzard to prevent such actions.
My goal with this guide is to help people who are uncomfortable with the current crosshair options in Overwatch until Blizzard implements a better system themselves (I'm sure they will eventually), without affecting the game in an invasive or obtrusive manner. If I am notified that any of this is explicitly against Blizzard's wishes I will immediately remove this guide from the sites I have posted it (Battlenet, Reddit), and cease using this method with my own account/copy of Overwatch.
THE GUIDE:
At first glance this guide may seem lengthy, but the process is quite simple.
I've tried to explain everything in detail to avoid confusion.
IMHO this is by far the easiest method to apply a fullscreen cross-hair overlay.
1. Download and install Overwolf: www.overwolf.com/
No need to create an online account, the program functions perfectly well without one.
2. Download the latest release of the Crosshair extension as an .OPK file:
https://github.com/OverwolfApps/OW-crosshair/releases/
3. In Overwolf click on Settings (either the wrench icon on the dock, or the Settings button on the tray icon menu).
4. Go to the Support tab, you will see a link labelled “Development Options”. Click that.
5. Click on “Install extension...” and select the OPK file you just downloaded.
Once installed you should set your preferred shortcut keys in the 'hotkeys' tab, and other settings such as the FPS overlay in the fps tab (I turn it off as Overwatch is already capable of displaying fps and other peformance info.)
Click the save button on the settings window to close it.
6. Right click on the overwolf tray icon and select appstore. Switch to the 'My Library' tab and check if Overwatch has been automatically detected by Overwolf. If you do not see it in the list of games, click '+add game', browse to your Overwatch directory and select the executable 'overwatch.exe'.
Close the appstore window.
7. Launch Overwatch through battlenet (Launching through overwolf will not automatically log in to your battnet account)
8. Set your crosshair in-game to 'short crosshairs' then enter a custom game without bots (or with, this is just for the purpose of aligning the crosshair).
9. Bring up the overwolf overlay (by default a small tab on the left of the screen) by pressing your selected hotkey. Hover over the tab and launch X-hair. http://imgur.com/a/BURDv

10. select a crosshair from the drop down menu or choose custom and browse for a file (must be transparent .png image, you can find numerous pre-made crosshairs with google search or make your own. For this guide I'm using a custom small green cross)
11. Click the re-centre button and use the micro-ajust arrows to align the cross-hairs. http://imgur.com/a/2w2TO
12. Click the select button to apply the overlay. If the crosshair jumps upwards a few pixels (mine did), you'll have to alt-tab and close the x-hair extension on the taskbar, then maximise overwatch and repeat steps 9-11, this time compensating for the sight jump. I did this by clicking the micro-ajust down arrow twice, which looks like this: http://imgur.com/a/u35yR
This time when you click select the crosshair should jump into perfect alignment. which looks like this: http://imgur.com/a/4Ja7i
13. Set the overwatch crosshair to 'small dot', which will be obscured if you use a custom crosshair similar to mine.
End result: http://imgur.com/a/RzpB9
I hope this brings relief to everyone with a particular preference for their crosshair. I've been using the same green cross in every multiplayer shooter for over a decade, this is the first time I've had to resort to an overlay.
BUG FIX
Some users report that running Overwatch on your secondary screen with Overwolf will block your game inputs. In order to fix this issue you just have to move the game to your main monitor.

Thanks for the guide

Would love an actual response of Jeff J. Kerplank on this.

bannable?

Don't know if OVerwolf's bannable or not, but Blizz should add 'draw-your-own-damn-crosshair' as an option, cause I'd definitely use it.
Jeff Kapoeira's input greatly appreciated

Crosshair overlay can't be bannable. I believe there are even monitors you can buy from big time, reputable manufacturers that have this feature built right in.

Crosshair overlay can't be bannable. I believe there are even monitors you can buy from big time, reputable manufacturers that have this feature built right in.

Only Asus moniters have this feature as far as I know, and only on a few models: MG248Q, MG28UQ and MG24UQ.
I thought of buying one just for the crosshair feature haha, but I already have a decent gaming monitor and I'm not made of money.

02/08/2017 08:12 PMPosted by ZOOPS

My PG279Q has it. I don't use it because it is ulgy AF and overdone. I will give a try to Overwolf, just keep in mind that this software in harvesting your personal data like crazy, like the big-data of gaming industry.

They only gather personal info if you make an account, and if you use the browser overlay they have access to your history.
I don't have an online account and I don't use the browser, so I don't see any of this as an issue. You could of course block the program in windows firewall from accessing the internet, as it functions fine without it.

Good tip.

Hey there Overwolf support here,
The X-Hair app is an external app developed outside of Overwolf and is not authorized by us.
Thus, it is not supported by Overwolf and keep in mind that using it is at your own risk.
Overwolf support manager
Or Manor

virustotal scans reveal nothing wrong with the file/archive, however it is important to Know that even they can't detect everything (seen it for myself).
I'm currently working on a simple json edit (with an online editor) and re-adding it into the archive (via 7zip portable). This will work as a form of duplication for the extension, allowing the users to toggle between 2 or 3 crosshairs (I will only make 2 duplicates, it's not hard but I will try to edit the icons and a few x-hairs myself as well).
Will return with an edited post or a new reply - in the meantime, got any suggestions for uploading the files? (or shall I use github as well?)
EDIT: By the way, it is possible to use animated gifs renamed to .png and they will playback (you can easily make them with GIMP) - transparency is of course supported

Not to belittle your efforts in any way, but genuinely curious: What is the purpose of all this? Just a different aesthetic for your crosshair?

this is an awesome tutorial thanks, although its use is limited in OW. i can see so much possibilities in other games such as battlefield 1, or pubg.
you can even create a range finder.
like this.
https://res.cloudinary.com/pvplive/image/upload/fl_lossy,f_auto/article_images/vssscope2.PNG
i wish the crosshair screen could be binded to right click or something.
thanks though, awesome tutorial

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