Child image builds have often desynchronized the building progress bar, as the extension did not detect when a child image started building as part of a build process. This caused the progress bar to look like it was stuck on 95% while the parent image completed its build process.
With sysbuild, this problem is accentuated, and we decided to revamp the progress bar mechanism for multi-image builds. Now, when building a configuration with child images, each child image will show up with its own progress bar to avoid the problem of the main build hanging.