There is no DrawBorder method in fabric api [1.21.9-rc1]

This doesn't seem like a problem with fabric api. Could you share more information about what the problem is and how you expect fabric to be able to solve it?

It's replaced by net.minecraft.client.gui.DrawContext#drawStrokedRectangle. Not a Fabric API issue.

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Thanks

There is an issue where it only draws in gui screens. If you try and draw it into the hud it doesn’t work.

It's replaced by net.minecraft.client.gui.DrawContext#drawStrokedRectangle. Not a Fabric API issue.

drawStrokedRectangle does not appear to scale to the Matrix though.

Look at the implementation, it submits the draw calls to be drawn later. It won't work with HUD rendering. You can make a helper method by using DrawContext#fill.

Oddly enough, this was reverted back on 1.21.10.