vehicle part descriptions: properly handle the scroll limit

limit scrolling such that you can't scroll if all the descriptions
fit it the window, and you can't scroll down past the point that
the remaining descriptions fit in the window, and you can't scroll
up past the first description.

Method hopefully documented adequately in the function, because I
may need to reuse it in the future.

src/veh_interact.cpp

@@ -1581,7 +1581,7 @@ void veh_interact::move_cursor( int dx, int dy, int dstart_at )
     ddx += dy;
     ddy -= dx;
     if( dx || dy ) {
-       start_at = 0;
+       start_limit = 0;
     } else {
        start_at += dstart_at;
     }
@@ -1609,7 +1609,7 @@ void veh_interact::move_cursor( int dx, int dy, int dstart_at )
     wrefresh( w_parts );
 
     werase( w_msg );
-    veh->print_vparts_descs( w_msg, getmaxy( w_msg ), getmaxx( w_msg ), cpart, start_at );
+    veh->print_vparts_descs( w_msg, getmaxy( w_msg ), getmaxx( w_msg ), cpart, start_at, start_limit );
     wrefresh( w_msg );
 
     can_mount.clear();

src/veh_interact.h

@@ -55,7 +55,9 @@ class veh_interact
 
         int ddx = 0;
         int ddy = 0;
+        /* starting offset for vehicle parts description display and max offset for scrolling */
         int start_at = 0;
+        int start_limit = 0;
         const vpart_info *sel_vpart_info = nullptr;
         char sel_cmd = ' '; //Command currently being run by the player
 

src/vehicle.cpp

@@ -2892,9 +2892,10 @@ int vehicle::print_part_list( const catacurses::window &win, int y1, const int m
  * @param width The width of the window.
  * @param &p The index of the part being examined.
  * @param start_at Which vehicle part to start printing at.
+ * @param start_limit the part index beyond which the display is full
  */
 void vehicle::print_vparts_descs( const catacurses::window &win, int max_y, int width, int &p,
-                                  int &start_at ) const
+                                  int &start_at, int &start_limit ) const
 {
     if( p < 0 || p >= ( int )parts.size() ) {
         return;
@@ -2904,9 +2905,23 @@ void vehicle::print_vparts_descs( const catacurses::window &win, int max_y, int
     std::ostringstream msg;
 
     int lines = 0;
-    /* never start before the start of the list */
-    /* guess the number of entries that can fit in the window, and don't scroll past that */
-    start_at = std::min( std::max( start_at, 0 ), max_y / 6 );
+    /*
+     * start_at and start_limit interaction is little tricky
+     * start_at and start_limit start at 0 when moving to a new frame
+     * if all the descriptions are displayed in the window, start_limit stays at 0 and
+     *    start_at is capped at 0 - so no scrolling at all.
+     * if all the descriptions aren't displayed, start_limit jumps to the last displayed part
+     *    and the next scrollthrough can start there - so scrolling down happens.
+     * when the scroll reaches the point where all the remaining descriptions are displayed in
+     *    the window, start_limit is set to start_at again.
+     * on the next attempted scrolldown, start_limit is set to the nth item, and start_at is
+     *    capped to the nth item, so no more scrolling down.
+     * start_at can always go down, but never below 0, so scrolling up is only possible after
+     *    some scrolling down has occurred.
+     * important! the calling function needs to track p, start_at, and start_limit, and set
+     *    start_limit to 0 if p changes.
+     */
+    start_at = std::max( 0, std::min( start_at, start_limit ) );
     if( start_at ) {
            msg << "<color_yellow>" << "<  " << _( "More parts here..." ) << "</color>\n";
            lines += 1;
@@ -2924,8 +2939,10 @@ void vehicle::print_vparts_descs( const catacurses::window &win, int max_y, int
         if( lines + new_lines <= max_y ) {
            msg << possible_msg.str();
            lines += new_lines;
+	   start_limit = start_at;
         } else {
            msg << "<color_yellow>" << _( "More parts here..." ) << "  >" << "</color>\n";
+           start_limit = i;
            break;
         }
     }

src/vehicle.h

@@ -581,7 +581,7 @@ class vehicle
 
         // Vehicle parts descriptions - descriptions for all the parts on a single tile
         void print_vparts_descs( const catacurses::window &win, int max_y, int width, int &p,
-                                 int &start_at ) const;
+                                 int &start_at, int &start_limit ) const;
 
     /**
      *  Operate vehicle controls