diff --git a/.github/workflows/testpr.yaml b/.github/workflows/testpr.yaml
index e71f74ab..dd008b76 100644
--- a/.github/workflows/testpr.yaml
+++ b/.github/workflows/testpr.yaml
@@ -77,6 +77,9 @@ jobs:
         run: pixi run test
 
   build:
+    permissions:
+      actions: write  # drop this pull request's superseded build caches
+      contents: read
     needs: plan
     if: needs.plan.outputs.distros != '[]'
     strategy:
@@ -242,12 +245,31 @@ jobs:
         if [ -n "$newest" ]; then echo "Removing $newest"; rm -f "$newest"; fi
 
     - name: Save build cache
+      id: save-cache
       if: ${{ always() && contains(fromJSON('["success","failure","cancelled"]'), steps.build-recipes.outcome) }}
       uses: actions/cache/save@v5
       with:
         path: ${{ env.CACHE_DIR }}
         key: ${{ runner.os }}-${{ matrix.distro }}-${{ matrix.target.platform }}-pr-${{ github.event.pull_request.number }}-${{ github.run_id }}-${{ github.run_attempt }}
 
+    # Only the newest cache of a pull request/distribution/platform is restored; the ones
+    # of earlier runs and attempts only push others out of the repository's 10 GB, so
+    # full rebuilds lost their progress. (Read-only token on forks: then this is skipped.)
+    - name: Drop this pull request's superseded build caches
+      if: ${{ always() && steps.save-cache.outcome == 'success' && github.event.pull_request.number }}
+      continue-on-error: true
+      shell: bash
+      env:
+        GH_TOKEN: ${{ github.token }}
+        PREFIX: ${{ runner.os }}-${{ matrix.distro }}-${{ matrix.target.platform }}-pr-${{ github.event.pull_request.number }}-
+        KEEP: ${{ runner.os }}-${{ matrix.distro }}-${{ matrix.target.platform }}-pr-${{ github.event.pull_request.number }}-${{ github.run_id }}-${{ github.run_attempt }}
+      run: |
+        gh cache list --repo "$GITHUB_REPOSITORY" --key "$PREFIX" --limit 100 --json id,key \
+          --jq ".[] | select(.key != \"$KEEP\") | .id" |
+        while read -r id; do
+          gh cache delete "$id" --repo "$GITHUB_REPOSITORY" && echo "deleted cache $id"
+        done
+
     - name: Generate the staged build workflows (catches post-merge problems)
       shell: bash -l {0}
       run: |
@@ -290,12 +312,16 @@ jobs:
             echo "$marker"
             echo "### ABI check (linux-64)"
             echo
+            cat "${files[@]}"
+            echo "<details><summary>About this check</summary>"
+            echo
             echo "Each package this pull request built, compared with its newest released build (\`tools/abi_check.py\`, libabigail):"
             echo "- **compatible** / **additions**: the package can be bumped on its own;"
             echo "- **soname** / **incompatible**: the packages that depend on it need a rebuild (build number bump) in the same pull request."
             echo
-            cat "${files[@]}"
             echo "Exported symbols are compared (the released packages are stripped, so struct layouts aren't); unchanged installed headers rule out layout and inline changes, changed ones are listed for review. Dependents to rebuild are the released ones linking to a changed library. A changed pin is part of the comparison."
+            echo
+            echo "</details>"
           } > body.md
           if [ -n "$existing" ]; then
             gh api -X PATCH "repos/$REPO/issues/comments/$existing" -F body=@body.md > /dev/null
diff --git a/tools/abi_check.py b/tools/abi_check.py
index f2860d93..3fcf15c3 100644
--- a/tools/abi_check.py
+++ b/tools/abi_check.py
@@ -505,9 +505,10 @@ def rebuild_snippet(results: dict, distro: str, build_number: int | None) -> lis
 
 
 def pr_summary(results: dict, distro: str, platform: str, channel: str, build_number: int | None = None) -> list[str]:
-    """Markdown for the pull-request comment: what needs attention as a table, the
-    rest collapsed. Packages without a release are left out; a distribution with
-    nothing to compare gets no section."""
+    """Markdown for the pull-request comment: one collapsed section per distribution,
+    with the counts in its summary line, open only when dependents need a rebuild.
+    Inside, what needs attention as a table, the rest collapsed again. Packages without
+    a release are left out; a distribution with nothing to compare gets no section."""
     def worst_first(item):
         verdict = item[1]["verdict"]
         return (-RANK.index(verdict) if verdict in RANK else 1, item[0])
@@ -518,21 +519,24 @@ def worst_first(item):
 
     if not attention and not rest:
         return []
-    lines = [f"#### {distro} {platform}", ""]
+    breaking = [n for n, r in attention if r["verdict"] in ("soname", "incompatible", "error")]
+    counts = [f"**{len(breaking)} need their dependents rebuilt**" if breaking else "",
+              f"{len(attention) - len(breaking)} to review (changed headers)" if len(attention) > len(breaking) else "",
+              f"{len(rest)} can be bumped on their own" if rest else ""]
+    lines = [f"<details{' open' if breaking else ''}><summary><b>{distro} {platform}</b>: "
+             f"{', '.join(c for c in counts if c)}</summary>", ""]
     if attention:
         lines += TABLE_HEADER + [_row(n, r) for n, r in attention] + [""]
         lines += rebuild_snippet(dict(attention), distro, build_number)
     if rest:
-        counts: dict[str, int] = {}
+        verdicts: dict[str, int] = {}
         for _, res in rest:
-            counts[res["verdict"]] = counts.get(res["verdict"], 0) + 1
-        summary = ", ".join(f"{v}: {c}" for v, c in sorted(counts.items()))
-        if not attention:
-            lines += ["Nothing that needs a rebuild of other packages.", ""]
+            verdicts[res["verdict"]] = verdicts.get(res["verdict"], 0) + 1
+        summary = ", ".join(f"{v}: {c}" for v, c in sorted(verdicts.items()))
         lines += [f"<details><summary>{len(rest)} packages can be bumped on their own ({summary})</summary>", "",
                   *TABLE_HEADER, *[_row(n, r) for n, r in rest], "", "</details>", ""]
     where = " and ".join(f"{c}/{platform}" for c in ([channel] if isinstance(channel, str) else channel))
-    return lines + [f"Compared with the newest builds on {where}.", ""]
+    return lines + [f"Compared with the newest builds on {where}.", "", "</details>", ""]
 
 
 def main() -> int:
diff --git a/tools/tests/test_abi_check.py b/tools/tests/test_abi_check.py
index 7787897a..cf6ed388 100644
--- a/tools/tests/test_abi_check.py
+++ b/tools/tests/test_abi_check.py
@@ -46,6 +46,12 @@ def test_summary_puts_the_snippet_under_the_table():
     text = "\n".join(abi_check.pr_summary(results, "jazzy", "linux-64", ["https://x/test", "https://x/jazzy"], 26))
     assert text.index("| foo |") < text.index("bar:\n  build_number: 26") < text.index("<details>")
     assert "https://x/test/linux-64 and https://x/jazzy/linux-64" in text
+    # one section per distribution, open (with the count in the summary) when dependents need a rebuild
+    assert text.startswith("<details open><summary><b>jazzy linux-64</b>: **1 need their dependents rebuilt**, "
+                           "1 can be bumped on their own</summary>")
+    assert text.rstrip().endswith("</details>")
+    quiet = "\n".join(abi_check.pr_summary({"ok": results["ok"]}, "jazzy", "linux-64", "https://x/jazzy", 26))
+    assert quiet.startswith("<details><summary><b>jazzy linux-64</b>: 1 can be bumped on their own</summary>")
 
 
 def test_next_build_number_is_above_vinca_and_every_published_build(tmp_path, monkeypatch):