Add scroll margin to TOC keyboard navigation (toc_scroll_margin)

pdf_viewer/config.cpp

@@ -199,6 +199,7 @@ int SINGLE_MAIN_WINDOW_MOVE[2] = { -1, -1 };
 bool ENABLE_EXPERIMENTAL_FEATURES = false;
 bool CREATE_TABLE_OF_CONTENTS_IF_NOT_EXISTS = true;
 int MAX_CREATED_TABLE_OF_CONTENTS_SIZE = 5000;
+int TOC_SCROLL_MARGIN = 3;
 bool FORCE_CUSTOM_LINE_ALGORITHM = false;
 float OVERVIEW_SIZE[2] = { 0.8f, 0.4f };
 float OVERVIEW_OFFSET[2] = { 0.0f, 0.0f };
@@ -1130,6 +1131,7 @@ ConfigManager::ConfigManager(const Path& default_path, const Path& auto_path, co
     add_int(L"status_bar_font_size", &STATUS_BAR_FONT_SIZE, IntExtras{1, 100});
     add_int(L"text_summary_context_size", &TEXT_SUMMARY_CONTEXT_SIZE, IntExtras{1, 100});
     add_int(L"max_created_toc_size", &MAX_CREATED_TABLE_OF_CONTENTS_SIZE, IntExtras{1, 100000});
+    add_int(L"toc_scroll_margin", &TOC_SCROLL_MARGIN, IntExtras{0, 100});
     add_int(L"prerendered_page_count", &PRERENDERED_PAGE_COUNT, IntExtras{0, 10});
     add_int(L"reload_interval_miliseconds", &RELOAD_INTERVAL_MILISECONDS, IntExtras{0, 10000});
     add_ivec2(L"main_window_size", MAIN_WINDOW_SIZE);

pdf_viewer/prefs.config

@@ -179,6 +179,11 @@ create_table_of_contents_if_not_exists 1
 # Limits the maximum size of created table of contents
 max_created_toc_size 5000
 
+# Number of rows of context to keep visible above/below the current selection
+# when navigating the table of contents with the keyboard. Set to 0 to restore
+# the old behavior (scroll the bare minimum needed to keep the selection visible).
+toc_scroll_margin 3
+
 # Warn the user on the command line only when redefining keys inside
 # the same file. When set to 1, sioyek will warn when redefining keys
 # from other files also

pdf_viewer/ui.h

@@ -79,6 +79,7 @@ const int max_select_size = 100;
 extern bool SMALL_TOC;
 extern bool MULTILINE_MENUS;
 extern bool TOUCH_MODE;
+extern int TOC_SCROLL_MARGIN;
 
 
 class HierarchialSortFilterProxyModel : public QSortFilterProxyModel {
@@ -173,6 +174,51 @@ class BaseSelectorWidget : public QWidget {
 
 };
 
+// QTreeView (and QAbstractItemView in general) only scrolls the bare minimum
+// needed to bring the current row into view (ScrollHint::EnsureVisible with no
+// margin). That means keyboard navigation can push the selection all the way to
+// the last visible row with no look-ahead, and when reversing direction the
+// viewport doesn't budge until the selection reaches the edge of whatever is
+// already on screen.
+//
+// This subclass keeps exactly `TOC_SCROLL_MARGIN` rows of context visible
+// above/below the current row at all times, by additionally requesting
+// visibility for an index that many steps above/below the target before
+// scrolling to the target itself. Because this re-asserts the same fixed
+// margin on every single step, continuous movement toward an edge ends up
+// scrolling the view exactly one row per keypress -- the selection stays
+// pinned at a fixed screen offset while the list moves underneath it. The
+// margin size itself never varies, unlike a recentering/chunked approach.
+class SioyekTreeView : public QTreeView {
+public:
+    using QTreeView::QTreeView;
+
+protected:
+    QModelIndex step_index(const QModelIndex& from, int n, bool down) const {
+        QModelIndex cur = from;
+        for (int i = 0; i < n; i++) {
+            QModelIndex next = down ? indexBelow(cur) : indexAbove(cur);
+            if (!next.isValid()) break;
+            cur = next;
+        }
+        return cur;
+    }
+
+    void scrollTo(const QModelIndex& index, ScrollHint hint = EnsureVisible) override {
+        if (!index.isValid() || TOC_SCROLL_MARGIN <= 0) {
+            QTreeView::scrollTo(index, hint);
+            return;
+        }
+
+        QModelIndex above = step_index(index, TOC_SCROLL_MARGIN, false);
+        QModelIndex below = step_index(index, TOC_SCROLL_MARGIN, true);
+
+        QTreeView::scrollTo(below, EnsureVisible);
+        QTreeView::scrollTo(above, EnsureVisible);
+        QTreeView::scrollTo(index, hint);
+    }
+};
+
 template<typename T>
 class FilteredTreeSelect : public BaseSelectorWidget {
 private:
@@ -189,7 +235,7 @@ class FilteredTreeSelect : public BaseSelectorWidget {
     FilteredTreeSelect(bool fuzzy, QStandardItemModel* item_model,
         std::function<void(const std::vector<int>&)> on_done,
         MainWidget* parent,
-        std::vector<int> selected_index) : BaseSelectorWidget(new QTreeView(), fuzzy, item_model, parent),
+        std::vector<int> selected_index) : BaseSelectorWidget(new SioyekTreeView(), fuzzy, item_model, parent),
         on_done(on_done)
     {
         auto index = QModelIndex();