When your engineers merge the pull request, or close it without merging, the canvas keeps its frames but can no longer change code. To keep building, duplicate the canvas: the copy gets a branch and a pull request of its own.
Before you start
A canvas whose pull request was merged or closed in GitHub. The Portals on it are dimmed.
Steps
Click the GitHub button in the canvas header. The popover says Merged, "Your changes were merged into your codebase. To keep working, duplicate this canvas." For a pull request closed without merging it says Closed.
Click Duplicate canvas. When the Canvas created message appears, click Open.
The copy opens, named after the original with (copy). It starts from your repository's branch, merged change included, and its header has no pull request yet. Work in it as usual. The first Sync to Github opens a new pull request, see Create a pull request.
If you keep clicking in the old canvas
Double-clicking a dimmed Portal, opening Build Mode, or sending a message in its chat opens the dialog Pull request merged: "The pull request for this canvas was merged. A canvas can only have one pull request, so you can no longer make code changes here. To keep working, duplicate this canvas." Duplicate canvas there does the same as the popover. Got it closes it. For a closed pull request the dialog is Pull request closed.
Open on Github in the popover opens the merged pull request, for example to read the review comments.
If it does not work
The canvas is still locked after the copy opened: that is the original. The copy is listed next to it on Home, with (copy) in its name.
A message starting with "Couldn't duplicate": the copy was not made. The messages are explained in Duplicate a canvas.