This post is quite old now but it seems like there’s still people coming sometimes with the same problem I had years ago.
By “outlines” I’m referring to those white edges appearing all around the parts. Those edges are just displays, usually for anchors (they appear under any anchored part) and they can be toggled on/off at the settings tab. No need to reset Studio settings, but if you want to do so or this method does not work, then go ahead, It will work.
It might be a plugin that you have. Some plugins tend to have features that are weird. Did you download a new plugin lately? What I would do would be to create a new game and see if the grid appears there too. If it did, I would disable every plugin and create another game to see if it still has the grids. If it doesn’t, then that means that it was one of your plugins. If it is one of your plugins, start enabling one by one and see which one causes the problem. If it isn’t plugins, then it might be a studio bug or something.
Nope, It isn’t a plugin, I didn’t download any plugins lately and, the new parts doesn’t generate the lines, even if they are created into the original game. Only the old parts that already where in the map have the lines.
They look like the grid lines in the view option, you may of enable them in studio in view by accident as you can turn them on?
I used the Show grid for many time and I didn’t have any issue. I turned off the grid now and any line disappeared.
Oh i just noticed its not the grid lines, my bad. Yeah I don’t know I haven’t come across that yet.
Pretty sure they’re outlines, will follow up after researching how it works.
EDIT:
Go to the properties of " Lighting " in the Explorer and there will be a " LegacyOutlines " box under the " Appearance " tab wherein you can disable and or enable it.
If you’re referring to the grid, this is a feature that you can toggle under the “View” tab in studio.
That didn’t work, they aren’t outlines
By “Lines”, are you referring to the pixelated edges?
So these “lines” as you call them are a result of how rendering in computer graphics works - since screens are made up of many square pixels in rows, when you see a curved line on the screen it’s actually just a bunch of squares combined together to look like a curve from a distance.
For more information, you can check the wiki on anti-aliasing here.
The pixelated edges can be made less noticable with anti-aliasing. Roblox supports anti-aliasing on graphics levels 9 and 10, which you can enable in studio’s settings under the “Rendering” tab. You can also set your graphics level from the escape menu when in a game.
Please note however, that if you are running Roblox on older/weaker hardware, your computer may not support antialiasing.
Just in case changing your studio quality doesn’t work and you use a large/4K monitor, you can change your high DPI settings on the Roblox Studio application. This fixes that issue for me. Start by going to the folder containing the application.
Anything worked guys 
This is what you’re looking for
Try reading @Noble_Draconian’s post #12 again:
It explains how to set it simply in your Studio Settings tab.
If you tried that, I think @flamenco_687 was referring to resetting the Studio settings. You can do this by uninstalling Studio and reinstalling it again. It puts everything back to the default settings.
sorry i meant to dele tat as when i figured it out 1 second later after i posted
Yup, that’s true, you must reset all the Studio settings, but there’s an easier way, I’ll modify the original post. At that time my english was pretty bad and nowadays I read the post and it’s pretty hard for me to understand my own comments.
