ui: Do not persistently hide stack with current equation

Add support for a "transient object" to be shown for context.
This object will be hidden the next time the stack is redrawn.

That way, we only see the equation when it's useful.

Fixes: #1077

Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>

src/solve.cc

@@ -591,6 +591,10 @@ MENU_BODY(SolvingMenu)
         ui.marker(k + 1 * ui.NUM_SOFTKEYS, L'?', false);
         ui.marker(k + 2 * ui.NUM_SOFTKEYS, L'▶', false);
     }
+
+    if (expression_p expr = expression::current_equation(false, true, true))
+        ui.transient_object(expr);
+
     return true;
 }
 

src/user_interface.cc

@@ -2600,10 +2600,8 @@ bool user_interface::draw_stack()
     draw_busy();
     uint top = HeaderFont->height() + 2;
     uint bottom = Stack.draw_stack();
-    if (menu_p m = menu())
-        if (m->type() == object::ID_SolvingMenu)
-            if (expression_p expr = expression::current_equation(false, true, true))
-                draw_object(expr, top, bottom);
+    if (object_p transient = transient_object())
+        draw_object(transient, top, bottom);
     draw_dirty(0, top, stack, LCD_H-1);
     draw_idle();
     dirtyStack = false;
@@ -2647,6 +2645,38 @@ bool user_interface::draw_object(object_p obj, uint top, uint bottom)
 }
 
 
+object_p user_interface::transient_object()
+// ----------------------------------------------------------------------------
+//   Return transient object to display if any
+// ----------------------------------------------------------------------------
+{
+    object_p result = editing;
+    if (result)
+    {
+        if (!rt.editing())
+            editing = nullptr;
+        else
+            result = nullptr;
+    }
+    return result;
+}
+
+
+bool user_interface::transient_object(object_p obj)
+// ----------------------------------------------------------------------------
+//   Set transient object to draw on screen
+// ----------------------------------------------------------------------------
+{
+    if (obj && !editing && !rt.editing())
+    {
+        editing = obj;
+        dirtyStack = true;
+        return true;
+    }
+    return false;
+}
+
+
 void user_interface::load_help(utf8 topic, size_t len)
 // ----------------------------------------------------------------------------
 //   Find the help message associated with the topic

src/user_interface.h

@@ -143,6 +143,9 @@ struct user_interface
     bool        draw_menus();
     bool        draw_cursor(int show, uint ncursor);
 
+    object_p    transient_object();
+    bool        transient_object(object_p obj);
+
     modes       editing_mode()          { return mode; }
     int         stack_screen_bottom()   { return stack; }
     int         menu_screen_bottom()    { return menuHeight; }