Fix inconsistencies with Tab/Backspace handling in menus (fltk#1157)

src/Fl_Menu.cxx

@@ -698,40 +698,44 @@ static void setitem(int m, int n) {
 }
 
 static int forward(int menu) { // go to next item in menu menu if possible
-  menustate &pp = *p;
-  // Fl_Menu_Button can generate menu=-1. This line fixes it and selects the first item.
-  if (menu==-1)
+  // `menu` is -1 if no item is currently selected, so use the first menu
+  if (menu < 0)
     menu = 0;
+  menustate &pp = *p;
   menuwindow &m = *(pp.p[menu]);
   int item = (menu == pp.menu_number) ? pp.item_number : m.selected;
-  int item0 = item;
+  bool wrapped = false;
   do {
     while (++item < m.numitems) {
       const Fl_Menu_Item* m1 = m.menu->next(item);
       if (m1->activevisible()) {setitem(m1, menu, item); return 1;}
     }
-    if (item0 == -1) break;
+    if (wrapped) break;
     item = -1;
+    wrapped = true;
   }
-  while (pp.menubar && Fl::event_key() == FL_Right);
+  while (Fl::event_key() != FL_Down);
   return 0;
 }
 
 static int backward(int menu) { // previous item in menu menu if possible
-  // `menu` is -1 if no item is currently selected, we return 0
-  if (menu<0)
-    return 0;
+  // `menu` is -1 if no item is currently selected, so use the first menu
+  if (menu < 0)
+    menu = 0;
   menustate &pp = *p;
   menuwindow &m = *(pp.p[menu]);
   int item = (menu == pp.menu_number) ? pp.item_number : m.selected;
+  bool wrapped = false;
   do {
     while (--item >= 0) {
       const Fl_Menu_Item* m1 = m.menu->next(item);
       if (m1->activevisible()) {setitem(m1, menu, item); return 1;}
     }
+    if (wrapped) break;
     item = m.numitems;
+    wrapped = true;
   }
-  while (pp.menubar && Fl::event_key() == FL_Left);
+  while (Fl::event_key() != FL_Up);
   return 0;
 }
 
@@ -799,10 +803,7 @@ int menuwindow::handle_part1(int e) {
     switch (Fl::event_key()) {
     case FL_BackSpace:
     BACKTAB:
-      if (!backward(pp.menu_number)) {
-        pp.item_number = -1;
-        backward(pp.menu_number);
-      }
+      backward(pp.menu_number);
       return 1;
     case FL_Up:
       if (pp.menubar && pp.menu_number == 0) {
@@ -815,12 +816,10 @@ int menuwindow::handle_part1(int e) {
       return 1;
     case FL_Tab:
       if (Fl::event_shift()) goto BACKTAB;
+      if (pp.menubar && pp.menu_number == 0) goto RIGHT;
     case FL_Down:
       if (pp.menu_number || !pp.menubar) {
-        if (!forward(pp.menu_number) && Fl::event_key()==FL_Tab) {
-          pp.item_number = -1;
-          forward(pp.menu_number);
-        }
+        forward(pp.menu_number);
       } else if (pp.menu_number < pp.nummenus-1) {
         forward(pp.menu_number+1);
       }