Help+LibManual: Open sibling page for subsections

Clicking on a subsection now displays the sibling page, which is
intended to be the main page for that section.

Userland/Applications/Help/ManualModel.cpp

@@ -58,9 +58,9 @@ Optional<String> ManualModel::page_path(const GUI::ModelIndex& index) const
     if (!index.is_valid())
         return {};
     auto* node = static_cast<Manual::Node const*>(index.internal_data());
-    if (!node->is_page())
+    auto page = node->document();
+    if (!page)
         return {};
-    auto* page = static_cast<Manual::PageNode const*>(node);
     auto path = page->path();
     if (path.is_error())
         return {};

Userland/Libraries/LibManual/Node.h

@@ -26,6 +26,7 @@ public:
     virtual bool is_page() const { return false; }
     virtual bool is_open() const { return false; }
     virtual ErrorOr<String> path() const = 0;
+    virtual PageNode const* document() const = 0;
 
     // Backend for the command-line argument format that Help and man accept. Handles:
     // [/path/to/documentation.md] (no second argument)

Userland/Libraries/LibManual/PageNode.h

@@ -27,6 +27,7 @@ public:
     virtual Node const* parent() const override;
     virtual ErrorOr<String> name() const override { return m_page; };
     virtual bool is_page() const override { return true; }
+    virtual PageNode const* document() const override { return this; };
 
     virtual ErrorOr<String> path() const override;
 

Userland/Libraries/LibManual/SectionNode.h

@@ -33,6 +33,7 @@ public:
     virtual ErrorOr<String> name() const override;
     String const& section_name() const { return m_section; }
     virtual ErrorOr<String> path() const override;
+    virtual PageNode const* document() const override { return nullptr; }
 
     virtual bool is_open() const override { return m_open; }
     void set_open(bool open);

Userland/Libraries/LibManual/SubsectionNode.cpp

@@ -5,6 +5,8 @@
  */
 
 #include "SubsectionNode.h"
+#include "PageNode.h"
+#include <AK/TypeCasts.h>
 
 namespace Manual {
 
@@ -16,6 +18,24 @@ SubsectionNode::SubsectionNode(NonnullRefPtr<SectionNode> parent, StringView nam
 
 Node const* SubsectionNode::parent() const { return m_parent; }
 
+PageNode const* SubsectionNode::document() const
+{
+    auto maybe_siblings = parent()->children();
+    if (maybe_siblings.is_error())
+        return nullptr;
+    auto siblings = maybe_siblings.release_value();
+    for (auto const& sibling : siblings) {
+        if (&*sibling == this)
+            continue;
+        auto sibling_name = sibling->name();
+        if (sibling_name.is_error())
+            continue;
+        if (sibling_name.value() == m_name && is<PageNode>(*sibling))
+            return static_cast<PageNode*>(&*sibling);
+    }
+    return nullptr;
+}
+
 ErrorOr<String> SubsectionNode::name() const { return m_name; }
 
 ErrorOr<String> SubsectionNode::path() const

Userland/Libraries/LibManual/SubsectionNode.h

@@ -19,6 +19,7 @@ public:
     virtual Node const* parent() const override;
     virtual ErrorOr<String> path() const override;
     virtual ErrorOr<String> name() const override;
+    virtual PageNode const* document() const override;
 
 protected:
     NonnullRefPtr<SectionNode> m_parent;