diff --git a/controller/internal/web/nav_accordion_test.go b/controller/internal/web/nav_accordion_test.go
new file mode 100644
index 0000000..1d09d41
--- /dev/null
+++ b/controller/internal/web/nav_accordion_test.go
@@ -0,0 +1,137 @@
+package web
+
+import (
+	"regexp"
+	"strings"
+	"testing"
+)
+
+// v0.146.0 nav polish. The sidebar's collapsible groups are a PROGRESSIVE ENHANCEMENT: the server
+// marks the group owning the active page with .is-open, so the correct group is already open before
+// any JS runs and stays open if JS never runs at all. That server-side half is what these tests
+// pin — the part a browser screenshot would confirm only for whichever page happened to be open.
+//
+// Red-proof: drop the `{{if $storageOpen}} is-open{{end}}` (or the matching aria-expanded) from
+// layout.html and TestNavGroup_ActiveGroupRendersOpenServerSide fails.
+
+// navGroupRe finds a sidebar group <li> and captures its class list and the id of the group it owns.
+var navGroupRe = regexp.MustCompile(`<li class="(nav-group[^"]*)">\s*<button type="button" class="(nav-group-toggle[^"]*)" aria-expanded="(true|false)" aria-controls="(nav-group-[a-z]+)"`)
+
+type navGroup struct {
+	liClass, btnClass, ariaExpanded, id string
+}
+
+func parseNavGroups(t *testing.T, html string) map[string]navGroup {
+	t.Helper()
+	out := map[string]navGroup{}
+	for _, m := range navGroupRe.FindAllStringSubmatch(html, -1) {
+		out[m[4]] = navGroup{liClass: m[1], btnClass: m[2], ariaExpanded: m[3], id: m[4]}
+	}
+	if len(out) == 0 {
+		t.Fatal("no sidebar nav groups parsed — the accordion markup or this pattern has drifted")
+	}
+	return out
+}
+
+// renderNavFor renders any page through the shared layout so the sidebar is exercised for real,
+// rather than asserting against a hand-built fragment.
+func renderNavFor(t *testing.T, page string) string {
+	t.Helper()
+	return renderBackupPage(t, "backups", map[string]interface{}{
+		"Page": page, "Title": "teszt",
+		"Backup":      nil,
+		"GuestBackup": map[string]interface{}{"Available": false, "Note": "n/a"},
+	})
+}
+
+func TestNavGroup_ActiveGroupRendersOpenServerSide(t *testing.T) {
+	// Every sub-page, mapped to the group that must be open when it is the active page.
+	cases := map[string]string{
+		"storage":         "nav-group-storage",
+		"storage-network": "nav-group-storage",
+		"backups":         "nav-group-backups",
+		"backups-remote":  "nav-group-backups",
+		"backups-apps":    "nav-group-backups",
+		"backups-restore": "nav-group-backups",
+		"sharing":         "nav-group-sharing",
+	}
+
+	for page, wantOpen := range cases {
+		groups := parseNavGroups(t, renderNavFor(t, page))
+
+		openCount := 0
+		for id, g := range groups {
+			isOpen := strings.Contains(g.liClass, "is-open")
+			if isOpen {
+				openCount++
+			}
+			if id == wantOpen {
+				if !isOpen {
+					t.Errorf("page %q: group %s should render open, class was %q", page, id, g.liClass)
+				}
+				if g.ariaExpanded != "true" {
+					t.Errorf("page %q: group %s open but aria-expanded=%q", page, id, g.ariaExpanded)
+				}
+				// The header must also read as active, or the open group looks unselected.
+				if !strings.Contains(g.btnClass, "active") {
+					t.Errorf("page %q: group %s open but its toggle is not .active (%q)", page, id, g.btnClass)
+				}
+			} else {
+				if isOpen {
+					t.Errorf("page %q: group %s should be closed, class was %q", page, id, g.liClass)
+				}
+				if g.ariaExpanded != "false" {
+					t.Errorf("page %q: group %s closed but aria-expanded=%q", page, id, g.ariaExpanded)
+				}
+			}
+		}
+		// "Exactly one open" is the whole contract — assert the count, not just the one we expected.
+		if openCount != 1 {
+			t.Errorf("page %q: %d groups rendered open, want exactly 1", page, openCount)
+		}
+	}
+}
+
+func TestNavGroup_PageOutsideAnyGroupOpensNothing(t *testing.T) {
+	// A top-level page (Vezérlőpult) belongs to no group; nothing should be forced open.
+	groups := parseNavGroups(t, renderNavFor(t, "dashboard"))
+	for id, g := range groups {
+		if strings.Contains(g.liClass, "is-open") {
+			t.Errorf("dashboard: group %s rendered open, want all closed", id)
+		}
+		if g.ariaExpanded != "false" {
+			t.Errorf("dashboard: group %s aria-expanded=%q, want false", id, g.ariaExpanded)
+		}
+	}
+}
+
+func TestNavGroup_EveryGroupLandingPageSurvivesAsASubItem(t *testing.T) {
+	// The group headers stopped being links in v0.146.0, which is only safe because each group's own
+	// landing page is ALSO its first sub-item. If someone removes one of those sub-links, that
+	// destination becomes unreachable from the sidebar — silently, since the header still looks
+	// clickable. This asserts the property the conversion depended on.
+	html := renderNavFor(t, "dashboard")
+	for _, href := range []string{"/storage", "/backups", "/sharing"} {
+		if !strings.Contains(html, `href="`+href+`"`) {
+			t.Errorf("no sidebar link to %q — a group landing page is unreachable now that its header is a <button>", href)
+		}
+	}
+}
+
+func TestNavGroup_HeaderIsARealButtonWithAControlsTarget(t *testing.T) {
+	html := renderNavFor(t, "storage")
+
+	// Keyboard/AT reachability rides on it being a real button, not a div with a click handler.
+	if strings.Contains(html, `<div class="nav-group-toggle`) {
+		t.Error("nav-group-toggle rendered as a div — keyboard and assistive-tech reachability would be simulated, not real")
+	}
+	for id, g := range parseNavGroups(t, html) {
+		if !strings.HasPrefix(g.btnClass, "nav-group-toggle") {
+			t.Errorf("group %s: toggle class %q", id, g.btnClass)
+		}
+		// aria-controls must point at a list that actually exists in the document.
+		if !strings.Contains(html, `id="`+g.id+`"`) {
+			t.Errorf("group %s: aria-controls=%q targets no element", id, g.id)
+		}
+	}
+}