Update 1: If you remove the import of the layout.css in ./src/components/layout.js, the bug doesn't appear anymore.
Update 2: The bug doesn't appear when using styled-components. Check the styled-components branch for the implementation.
To see the bug, run yarn build 3 times and check the build log.
Issue on Github: gatsbyjs/gatsby#33450
This code is also on CodeSandbox. You can run the yarn command in the bash console on the bottom right terminal: https://codesandbox.io/s/gatsby-first-inc-build-bug-83pbu
The first build needs to build all HTML files. Starting with the second build, only incremental builds should be done. As there is no change in data or source, the first incremental build shouldn't rebuild any HTML page.
The first build builts all HTML files. This is correct. The second file rebuilts all HTML files. This is incorrect. Only with the third and upcoming builds, no HTML pages are built anymore.
First build (full build):
success Building HTML renderer - 1.964s
success Building static HTML for pages - 0.389s - 3/3 12.85/s
Second build (first incremental build):
success Building HTML renderer - 1.712s
success Building static HTML for pages - 6.326s - 3/3<> 0.79/s
Third and upcoming builds:
success Building HTML renderer - 0.802s
info There are no new or changed html files to build.