Window: refactor center_selection/display_selection_at

Take the line we are intersted in as a parameter rather than getting
the main selection last char line.

src/commands.cc

@@ -129,7 +129,7 @@ void edit(CommandParameters params, Context& context)
 
         context.selections() = context.buffer().clamp({ line,  column });
         if (context.has_window())
-            context.window().center_selection();
+            context.window().center_line(context.selections().main().last().line);
     }
 }
 

src/normal.cc

@@ -224,7 +224,7 @@ void goto_commands(Context& context, int line)
         context.push_jump();
         select_coord<mode>(context.buffer(), LineCount{line - 1}, context.selections());
         if (context.has_window())
-            context.window().center_selection();
+            context.window().center_line(LineCount{line-1});
     }
     else
     {
@@ -331,18 +331,19 @@ void view_commands(Context& context, int param)
         if (key.modifiers != Key::Modifiers::None or not context.has_window())
             return;
 
+        LineCount cursor_line = context.selections().main().last().line;
         Window& window = context.window();
         switch (tolower(key.key))
         {
         case 'v':
         case 'c':
-            context.window().center_selection();
+            context.window().center_line(cursor_line);
             break;
         case 't':
-            context.window().display_selection_at(0);
+            context.window().display_line_at(cursor_line, 0);
             break;
         case 'b':
-            context.window().display_selection_at(window.dimensions().line-1);
+            context.window().display_line_at(cursor_line, window.dimensions().line-1);
             break;
         case 'h':
             context.window().scroll(-std::max<CharCount>(1, param));

src/window.cc

@@ -42,18 +42,15 @@ Window::~Window()
     m_options.unregister_watcher(*this);
 }
 
-void Window::display_selection_at(LineCount line)
+void Window::display_line_at(LineCount buffer_line, LineCount display_line)
 {
-    if (line >= 0 or line < m_dimensions.line)
+    if (display_line >= 0 or display_line < m_dimensions.line)
-    {
+        m_position.line = std::max(0_line, buffer_line - display_line);
-        auto cursor_line = selections().main().last().line;
-        m_position.line = std::max(0_line, cursor_line - line);
-    }
 }
 
-void Window::center_selection()
+void Window::center_line(LineCount buffer_line)
 {
-    display_selection_at(m_dimensions.line/2_line);
+    display_line_at(buffer_line, m_dimensions.line/2_line);
 }
 
 void Window::scroll(LineCount offset)

src/window.hh

@@ -33,8 +33,8 @@ public:
 
     const DisplayBuffer& display_buffer() const { return m_display_buffer; }
 
-    void center_selection();
+    void center_line(LineCount buffer_line);
-    void display_selection_at(LineCount line);
+    void display_line_at(LineCount buffer_line, LineCount display_line);
     void scroll(LineCount offset);
     void scroll(CharCount offset);
     void update_display_buffer();