[hotkeys] add ctrl-d/ctrl-u hotkeys for moving by half a page

Also fixed a bug that caused the bottom status line to
be updated with the wrong line number.

Related to #945

NEWS.md

@@ -5,6 +5,8 @@ Features:
   be toggled by pressing CTRL-X.  While in cursor mode, any
   operations that would normally work on the "top" line now
   operate on the selected line instead.
+* Added CTRL-D and CTRL-U hotkeys to move down/up by half
+  a page.
 
 ## lnav v0.11.1
 

src/keymaps/default-keymap.json

@@ -70,7 +70,13 @@
                     "command": ":reset-session"
                 },
                 "x18": {
-                    "command": ";UPDATE lnav_views SET movement = (CASE movement WHEN 'top' THEN 'cursor' ELSE 'top' END) WHERE name = (SELECT name FROM lnav_view_stack ORDER BY rowid DESC LIMIT 1)"
+                    "command": ";UPDATE lnav_views SET movement = (CASE movement WHEN 'top' THEN 'cursor' ELSE 'top' END) WHERE name = (SELECT name FROM lnav_top_view)"
+                },
+                "x04": {
+                    "command": ";UPDATE lnav_views SET top = top + (height / 2) WHERE name = (SELECT name FROM lnav_top_view)"
+                },
+                "x15": {
+                    "command": ";UPDATE lnav_views SET top = top - (height / 2) WHERE name = (SELECT name FROM lnav_top_view)"
                 },
                 "x3d": {
                     "command": ";UPDATE lnav_views SET paused = 1 - paused"

src/lnav.cc

@@ -739,6 +739,20 @@ clear_last_user_mark(listview_curses* lv)
     }
 }
 
+static void
+update_view_position(listview_curses* lv)
+{
+    lnav_data.ld_view_stack.top() | [lv](auto* top_lv) {
+        if (lv != top_lv) {
+            return;
+        }
+
+        lnav_data.ld_bottom_source.update_line_number(lv);
+        lnav_data.ld_bottom_source.update_percent(lv);
+        lnav_data.ld_bottom_source.update_marks(lv);
+    };
+}
+
 class lnav_behavior : public mouse_behavior {
 public:
     void mouse_event(int button, bool release, int x, int y) override
@@ -1304,12 +1318,7 @@ looper()
         lnav_data.ld_view_stack.push_back(&lnav_data.ld_views[LNV_LOG]);
 
         sb.push_back(clear_last_user_mark);
-        sb.push_back(bind_mem(&bottom_status_source::update_line_number,
-                              &lnav_data.ld_bottom_source));
-        sb.push_back(bind_mem(&bottom_status_source::update_percent,
-                              &lnav_data.ld_bottom_source));
-        sb.push_back(bind_mem(&bottom_status_source::update_marks,
-                              &lnav_data.ld_bottom_source));
+        sb.push_back(update_view_position);
         vsb.push_back(
             bind_mem(&term_extra::update_title, injector::get<term_extra*>()));
         vsb.push_back([](listview_curses* lv) {