diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index dc80c9bfe72..ea2cb1aa79d 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -32,50 +32,82 @@ jobs:
         id: checkout
         uses: actions/checkout@v3
 
-      - name: Prepare
+      - name: Update system packages
         id: prepare
         run: |
-          # Install test tools.
-          if apt-get update && apt-get install -y git python3-venv; then
-             # Debian-specific temporary code:
-             # Installation of python3-venv can be removed as soon as a
-             # base image with a release including #33822 is available
-             :
-          else
-             export PATH="build/bin:$PATH"
-             eval $(sage-print-system-package-command auto update)
-             eval $(sage-print-system-package-command auto --spkg --yes --no-install-recommends install git)
-          fi
-          # Reuse built SAGE_LOCAL contained in the Docker image 
-          ./bootstrap
-          ./configure --enable-build-as-root --prefix=/sage/local --with-sage-venv --enable-editable --enable-download-from-upstream-url
-      
+          export PATH="build/bin:$PATH"
+          eval $(sage-print-system-package-command auto update)
+          eval $(sage-print-system-package-command auto --spkg --yes --no-install-recommends install git)
+
+      - name: Add prebuilt tree as a worktree
+        id: worktree
+        run: |
+          set -ex
+          git config --global user.email "ci-sage@example.com"
+          git config --global user.name "Build & Test workflow"
+          git config --global --add safe.directory $(pwd)
+          # If actions/checkout downloaded our source tree using the GitHub REST API
+          # instead of with git (because do not have git installed in our image),
+          # we first make the source tree a repo.
+          if [ ! -d .git ]; then git init && git add -A && git commit --quiet -m "new"; fi
+          # Tag this state of the source tree "new". This is what we want to build and test.
+          git tag -f new
+          # Our container image contains a source tree in /sage with a full build of Sage.
+          # But /sage is not a git repository.
+          # We make /sage a worktree whose index is at tag "new".
+          # We then commit the current sources and set the tag "old". (This keeps all mtimes unchanged.)
+          # Then we update worktree and index with "git reset --hard new".
+          # (This keeps mtimes of unchanged files unchanged and mtimes of changed files newer than unchanged files.)
+          # Finally we reset the index to "old". (This keeps all mtimes unchanged.)
+          # The changed files now show up as uncommitted changes.
+          git worktree add --detach worktree-image
+          rm -rf /sage/.git && mv worktree-image/.git /sage/
+          rm -rf worktree-image && ln -s /sage worktree-image
+          if [ ! -f worktree-image/.gitignore ]; then cp .gitignore worktree-image/; fi
+          (cd worktree-image && git add -A && git commit --allow-empty -m "old" -a && git tag -f old && git reset --hard new && git reset old)
+
+      - name: Incremental build, test changed files (sage -t --new)
+        id: incremental
+        run: |
+          # Now re-bootstrap and build. The build is incremental because we were careful with the timestamps.
+          # We run tests with "sage -t --new"; this only tests the uncommitted changes.
+          ./bootstrap && make build && ./sage -t --new -p2
+        working-directory: ./worktree-image
+        env:
+          MAKE: make -j2
+          SAGE_NUM_THREADS: 2
+
       - name: Build and test modularized distributions
-        if: always() && steps.prepare.outcome == 'success'
+        if: always() && steps.worktree.outcome == 'success'
         run: make V=0 tox && make pypi-wheels
+        working-directory: ./worktree-image
         env:
           MAKE: make -j2
           SAGE_NUM_THREADS: 2
 
       - name: Set up node to install pyright
-        if: always() && steps.prepare.outcome == 'success'
+        if: always() && steps.worktree.outcome == 'success'
         uses: actions/setup-node@v3
         with:
           node-version: '12'
 
       - name: Install pyright
-        if: always() && steps.prepare.outcome == 'success'
+        if: always() && steps.worktree.outcome == 'success'
         # Fix to v232 due to bug https://github.com/microsoft/pyright/issues/3239
         run: npm install -g pyright@1.1.232
 
       - name: Static code check with pyright
-        if: always() && steps.prepare.outcome == 'success'
-        run: pyright    
-      
-      - name: Build
+        if: always() && steps.worktree.outcome == 'success'
+        run: pyright
+        working-directory: ./worktree-image
+
+      - name: Build (fallback to non-incremental)
         id: build
-        if: always() && steps.prepare.outcome == 'success'
-        run: make build
+        if: always() && steps.worktree.outcome == 'success' && steps.incremental.outcome != 'success'
+        run: |
+          set -ex
+          ./bootstrap && make doc-clean doc-uninstall sagelib-clean && git clean -fx src/sage && ./config.status && make build
+        working-directory: ./worktree-image
         env:
           MAKE: make -j2
           SAGE_NUM_THREADS: 2
@@ -85,27 +117,28 @@ jobs:
         run: |
           ../sage -python -m pip install coverage pytest-xdist
           ../sage -python -m coverage run -m pytest -c tox.ini --doctest-modules || true
-        working-directory: ./src
+        working-directory: ./worktree-image/src
         env:
           # Increase the length of the lines in the "short summary"
           COLUMNS: 120
 
-      - name: Test
-        if: always() && steps.build.outcome == 'success'
+      - name: Test all files (sage -t --all --long)
+        if: always() && (steps.incremental.outcome == 'success' || steps.build.outcome == 'success')
         run: |
           ../sage -python -m pip install coverage
-          ../sage -python -m coverage run ./bin/sage-runtests --all -p2 --random-seed=286735480429121101562228604801325644303
-        working-directory: ./src
+          ../sage -python -m coverage run ./bin/sage-runtests --all --long -p2 --random-seed=286735480429121101562228604801325644303
+        working-directory: ./worktree-image/src
 
       - name: Prepare coverage results
-        if: always() && steps.build.outcome == 'success'
+        if: always() && (steps.incremental.outcome == 'success' || steps.build.outcome == 'success')
         run: |
           ./venv/bin/python3 -m coverage combine src/.coverage/
           ./venv/bin/python3 -m coverage xml
           find . -name *coverage*
-      
+        working-directory: ./worktree-image
+
       - name: Upload coverage to codecov
-        if: always() && steps.build.outcome == 'success'
+        if: always() && (steps.incremental.outcome == 'success' || steps.build.outcome == 'success')
         uses: codecov/codecov-action@v3
         with:
-          files: ./coverage.xml
+          files: ./worktree-image/coverage.xml
diff --git a/.github/workflows/doc-build.yml b/.github/workflows/doc-build.yml
index 4be22210eee..01748c3c823 100644
--- a/.github/workflows/doc-build.yml
+++ b/.github/workflows/doc-build.yml
@@ -19,30 +19,94 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v3
 
-      - name: Prepare
+      - name: Update system packages
         run: |
-          apt-get update && apt-get install -y zip
-          # Reuse built SAGE_LOCAL contained in the Docker image
-          ./bootstrap
-          ./configure --enable-build-as-root --prefix=/sage/local --with-sage-venv --enable-download-from-upstream-url
+          apt-get update && apt-get install -y git zip
 
-      - name: Build
-        run: make doc-html
+      - name: Add prebuilt tree as a worktree
+        id: worktree
+        run: |
+          set -ex
+          git config --global user.email "ci-sage@example.com"
+          git config --global user.name "Build & Test workflow"
+          git config --global --add safe.directory $(pwd)
+          # If actions/checkout downloaded our source tree using the GitHub REST API
+          # instead of with git (because do not have git installed in our image),
+          # we first make the source tree a repo.
+          if [ ! -d .git ]; then git init && git add -A && git commit --quiet -m "new"; fi
+          # Tag this state of the source tree "new". This is what we want to build and test.
+          git tag -f new
+          # Our container image contains a source tree in /sage with a full build of Sage.
+          # But /sage is not a git repository.
+          # We make /sage a worktree whose index is at tag "new".
+          # We then commit the current sources and set the tag "old". (This keeps all mtimes unchanged.)
+          # Then we update worktree and index with "git reset --hard new".
+          # (This keeps mtimes of unchanged files unchanged and mtimes of changed files newer than unchanged files.)
+          # Finally we reset the index to "old". (This keeps all mtimes unchanged.)
+          # The changed files now show up as uncommitted changes.
+          git worktree add --detach worktree-image
+          rm -rf /sage/.git && mv worktree-image/.git /sage/
+          rm -rf worktree-image && ln -s /sage worktree-image
+          if [ ! -f worktree-image/.gitignore ]; then cp .gitignore worktree-image/; fi
+          (cd worktree-image && git add -A && git commit --allow-empty -m "old" -a && git tag -f old && git reset --hard new && git reset old)
+          # Keep track of changes to built HTML
+          (cd /sage/local/share/doc/sage/html/en && find . -name "*.html" | xargs sed -i '/class="sidebar-brand-text"/s/Sage [0-9a-z.]* /Sage dev /'; git init && (echo ".buildinfo"; echo ".inv") > .gitignore; git add -A && git commit --quiet -m "old")
+
+      - name: Incremental build
+        id: incremental
+        run: |
+          # Now re-bootstrap and build. The build is incremental because we were careful with the timestamps.
+          ./bootstrap && make build
+        working-directory: ./worktree-image
+        env:
+          MAKE: make -j2
+          SAGE_NUM_THREADS: 2
+
+      - name: Build (fallback to non-incremental)
+        id: build
+        if: always() && steps.worktree.outcome == 'success' && steps.incremental.outcome != 'success'
+        run: |
+          set -ex
+          make doc-clean doc-uninstall sagelib-clean && git clean -fx src/sage && ./config.status && make build
+        working-directory: ./worktree-image
+        env:
+          MAKE: make -j2
+          SAGE_NUM_THREADS: 2
+
+      - name: Build docs
+        id: docbuild
+        if: always() && (steps.incremental.outcome == 'success' || steps.build.outcome == 'success')
+        # Always non-incremental because of the concern that
+        # incremental docbuild may introduce broken links (inter-file references) though build succeeds
+        run: |
+          set -ex
+          make doc-clean doc-uninstall sagelib-clean && git clean -fx src/sage && ./config.status && make doc-html
+        working-directory: ./worktree-image
         env:
           MAKE: make -j2
           SAGE_NUM_THREADS: 2
 
       - name: Copy docs
+        id: copy
+        if: always() && steps.docbuild.outcome == 'success'
         run: |
+          set -ex
+          mkdir -p ./docs
+          # Create changelog
+          echo '## Preview of CHANGES.html'
+          (cd /sage/local/share/doc/sage/html/en && find . -name "*.html" | xargs sed -i '/class="sidebar-brand-text"/s/Sage [0-9a-z.]* /Sage dev /'; git diff --name-only) | tee ./docs/CHANGES.txt
+          (cd /sage/local/share/doc/sage/html/en && git diff; rm -rf .git) > ./docs/html.diff
+          echo '## Preview of html.diff'; head -n 400 ./docs/html.diff
+          (echo '<p><a href="html.diff">HTML diff</a>'; sed -E 's,(.*),<p><a href="\1">\1</a>,' ./docs/CHANGES.txt) > ./docs/CHANGES.html
           # For some reason the deploy step below cannot find /sage/...
           # So copy everything from there to local folder
           # We also need to replace the symlinks because netlify is not following them
-          mkdir -p ./docs
           cp -r -L /sage/local/share/doc/sage/html/en/* ./docs
           # Zip everything for increased performance
           zip -r docs.zip docs
 
       - name: Upload docs
+        if: always() && steps.copy.outcome == 'success'
         uses: actions/upload-artifact@v3
         with:
           name: docs
diff --git a/.github/workflows/doc-publish.yml b/.github/workflows/doc-publish.yml
index 538e77322fc..e86727e73ef 100644
--- a/.github/workflows/doc-publish.yml
+++ b/.github/workflows/doc-publish.yml
@@ -72,7 +72,7 @@ jobs:
           header: preview-comment
           recreate: true
           message: |
-            [Documentation preview for this PR](${{ steps.deploy-netlify.outputs.NETLIFY_URL }}) (built with commit ${{ steps.source-run-info.outputs.sourceHeadSha }}) is ready! :tada:
+            [Documentation preview for this PR](${{ steps.deploy-netlify.outputs.NETLIFY_URL }}) (built with commit ${{ steps.source-run-info.outputs.sourceHeadSha }}; [changes](${{ steps.deploy-netlify.outputs.NETLIFY_URL }}/CHANGES.html)) is ready! :tada:
 
       - name: Update deployment status PR check
         uses: myrotvorets/set-commit-status-action@v1.1.7
diff --git a/build/pkgs/ecl/dependencies b/build/pkgs/ecl/dependencies
index 51a953403e9..cda6316bf5a 100644
--- a/build/pkgs/ecl/dependencies
+++ b/build/pkgs/ecl/dependencies
@@ -1,4 +1,4 @@
-$(MP_LIBRARY) readline gc libffi info
+$(MP_LIBRARY) readline gc libffi
 
 ----------
 All lines of this file are ignored except the first.
diff --git a/build/pkgs/ecl/dependencies_order_only b/build/pkgs/ecl/dependencies_order_only
new file mode 100644
index 00000000000..55021245387
--- /dev/null
+++ b/build/pkgs/ecl/dependencies_order_only
@@ -0,0 +1 @@
+info
diff --git a/build/pkgs/maxima/dependencies b/build/pkgs/maxima/dependencies
index 55c7e0d8d14..fffb89e2050 100644
--- a/build/pkgs/maxima/dependencies
+++ b/build/pkgs/maxima/dependencies
@@ -1,4 +1,4 @@
-ecl info
+ecl
 
 ----------
 All lines of this file are ignored except the first.
diff --git a/build/pkgs/maxima/dependencies_order_only b/build/pkgs/maxima/dependencies_order_only
new file mode 100644
index 00000000000..55021245387
--- /dev/null
+++ b/build/pkgs/maxima/dependencies_order_only
@@ -0,0 +1 @@
+info
diff --git a/src/sage/doctest/control.py b/src/sage/doctest/control.py
index 450d9b607da..abaa33f6239 100644
--- a/src/sage/doctest/control.py
+++ b/src/sage/doctest/control.py
@@ -791,7 +791,7 @@ def add_files(self):
         # SAGE_ROOT_GIT can be None on distributions which typically
         # only have the SAGE_LOCAL install tree but not SAGE_ROOT
         if SAGE_ROOT_GIT is not None:
-            have_git = os.path.isdir(SAGE_ROOT_GIT)
+            have_git = os.path.exists(SAGE_ROOT_GIT)
         else:
             have_git = False
 
@@ -866,11 +866,12 @@ def all_doc_sources():
                 data = line.strip().split(' ')
                 status, filename = data[0], data[-1]
                 if (set(status).issubset("MARCU")
-                    and filename.startswith("src/sage")
-                    and (filename.endswith(".py") or
-                         filename.endswith(".pyx") or
-                         filename.endswith(".rst"))):
-                    self.files.append(os.path.relpath(opj(SAGE_ROOT,filename)))
+                        and filename.startswith("src/sage")
+                        and (filename.endswith(".py") or
+                             filename.endswith(".pyx") or
+                             filename.endswith(".rst"))
+                        and not skipfile(opj(SAGE_ROOT, filename), self.options.optional)):
+                    self.files.append(os.path.relpath(opj(SAGE_ROOT, filename)))
 
     def expand_files_into_sources(self):
         r"""
diff --git a/src/sage/doctest/test.py b/src/sage/doctest/test.py
index a36b3caf64f..60075a53578 100644
--- a/src/sage/doctest/test.py
+++ b/src/sage/doctest/test.py
@@ -251,17 +251,24 @@
 
 Even though the doctester master process has exited, the child process
 is still alive, but it should be killed automatically
-after the `die_timeout` given above (10 seconds)::
+after the ``die_timeout`` given above (10 seconds)::
 
     sage: pid = int(open(F).read())    # long time
     sage: time.sleep(2)                # long time
     sage: os.kill(pid, signal.SIGQUIT) # long time; 2 seconds passed => still alive
     sage: time.sleep(8)                # long time
-    sage: os.kill(pid, signal.SIGQUIT) # long time; 10 seconds passed => dead
+    sage: os.kill(pid, signal.SIGQUIT) # long time; 10 seconds passed => dead  # random
     Traceback (most recent call last):
     ...
     ProcessLookupError: ...
 
+If the child process is dead and removed, the last output should be as above.
+However, the child process interrupted its parent process (see
+``"interrupt_diehard.rst"``), and became an orphan process. Depending on the
+system, an orphan process may eventually become a zombie process instead of
+being removed, and then the last output would just be a blank. Hence the ``#
+random`` tag.
+
 Test a doctest failing with ``abort()``::
 
     sage: subprocess.call(["sage", "-t", "--warn-long", "0",    # long time