Disable the fold button when no run is long enough to fold

src/lib/collapse.test.ts

@@ -38,7 +38,14 @@ describe("buildRows", () => {
 
   it("does not collapse runs shorter than three", () => {
     const short = chain("top:p1", "p1:p2", "p2:root", "root");
-    expect(hashes(buildRows(short, new Set(["top"]), true, new Set()))).toHaveLength(4);
+    const r = buildRows(short, new Set(["top"]), true, new Set());
+    expect(hashes(r)).toHaveLength(4);
+    expect(r.collapsible).toBe(false);
+  });
+
+  it("reports when a run can collapse, whether collapsed or not", () => {
+    expect(buildRows(commits, labeled, false, new Set()).collapsible).toBe(true);
+    expect(buildRows(commits, labeled, true, new Set()).collapsible).toBe(true);
   });
 
   it("stops a run at merges, forks and labels", () => {

src/lib/collapse.ts

@@ -4,6 +4,8 @@ export interface RowsResult {
   rows: DisplayRow[];
   /** Commit hash → id of the run of plain commits it belongs to. */
   runOf: Map<string, string>;
+  /** Whether any run is long enough to collapse. */
+  collapsible: boolean;
 }
 
 /**
@@ -35,6 +37,7 @@ export function buildRows(
   const hidden = new Set<string>();
   /** First commit of a collapsed run → [last commit, number hidden]. */
   const jump = new Map<string, [string, number]>();
+  let collapsible = false;
 
   for (const c of commits) {
     if (!plain(c.hash) || plain(childOf.get(c.hash))) continue;
@@ -47,6 +50,7 @@ export function buildRows(
     }
     const id = c.hash;
     for (const h of run) runOf.set(h, id);
+    if (run.length >= 3) collapsible = true;
     if (run.length >= 3 && collapseAll !== toggled.has(id)) {
       for (const h of run.slice(1, -1)) hidden.add(h);
       jump.set(c.hash, [run[run.length - 1], run.length - 2]);
@@ -63,5 +67,5 @@ export function buildRows(
         : { commit: c, parents: c.parents.map((hash) => ({ hash, dotted: false })) },
     );
   }
-  return { rows, runOf };
+  return { rows, runOf, collapsible };
 }