A new layer in the UI Builder now starts with a border width of 0. It used to start with Roblox's 1px dark outline, which nearly every design then had to remove by hand.

Everything has to agree

A default like this touches a lot of places. The canvas, the Inspector's Border width, the Code tab and every export (Instance.new, React-Luau, Fusion and the model file) now agree on 0. The exports write the 0 out explicitly. If they left it out, Roblox's own default of 1 would fill in when the script ran, and the design in Studio would differ from the design in the builder.

The border's colour and mode are unchanged, so typing a width gives the same outline as before.

Two defaults, on purpose

A design that already exists keeps the widths it was made with. To clear them, select several layers and set Width to 0, which is one undo step.

Opening a Roblox model file is different. If a layer in the file says nothing about a border, the file means Roblox's default of 1, so the importer should give it 1. So gui-schema.js now records two values for BorderSizePixel: default: 0, what a new layer in the builder gets, and engineDefault: 1, what a file means when it's silent. rbxm-import.js uses the second for anything a file leaves out.

Mixing those two up would have been easy. If imported files had picked up the builder's 0, every imported layer would have quietly lost its outline. Tests in border-default.test.js cover both directions.