Fix Fl_Pack to support more box types.

Old code supported only frame styles, this code
can now handle box types with a background.

FL/Fl_Pack.H

@@ -21,6 +21,7 @@
 #define Fl_Pack_H
 
 #include <FL/Fl_Group.H>
+#include <FL/Fl_Rect.H>
 
 /**
   This widget was designed to add the functionality of compressing and
@@ -39,6 +40,12 @@
   resizable() widget is the last widget in the group it is extended to take
   the full available width or height, respectively, of the Fl_Pack group.
 
+  \note Fl_Pack is optimized to use a frame-only (`..._FRAME`) box type.
+    Box types with background graphics (`..._BOX`) generally work, but can be
+    slower witch a large number of children. Not all box types work well
+    with FL_Pack. Avoid irregular graphics like FL_DIAMOND_BOX and background
+    images.
+
   \note You can nest Fl_Pack widgets or put them inside Fl_Scroll widgets
     or inside other group widgets but their behavior can sometimes be
     <i>"surprising"</i>. This is partly due to the fact that Fl_Pack widgets
@@ -61,6 +68,7 @@ public:
   };
 
 protected:
+  void draw_filler_(const Fl_Rect& rect);
   void draw() override;
 
 public:

src/Fl_Pack.cxx

@@ -49,6 +49,22 @@ Fl_Pack::Fl_Pack(int X, int Y, int W, int H, const char *L)
   // type(VERTICAL); // already set like this
 }
 
+/** Draw a rectangular area as a background filler.
+  If the box type has a background (is not a frame), this will clip to the
+  rectangular area and draw the box. Otherwise, it will draw a rectangle
+  in widget color.
+  \param[in] rect fill this rectangle inside the widget bounds
+ */
+void Fl_Pack::draw_filler_(const Fl_Rect& rect) {
+  if (Fl::box_bg(box())) {
+    fl_push_clip(rect.x(), rect.y(), rect.w(), rect.h());
+    draw_box();
+    fl_pop_clip();
+  } else {
+    fl_rectf(rect.x(), rect.y(), rect.w(), rect.h(), color());
+  }
+}
+
 void Fl_Pack::draw() {
   int tx = x()+Fl::box_dx(box());
   int ty = y()+Fl::box_dy(box());
@@ -102,11 +118,10 @@ void Fl_Pack::draw() {
       }
       if (spacing_ && current_position>maximum_position && box() &&
         (X != o->x() || Y != o->y() || d&FL_DAMAGE_ALL)) {
-        fl_color(color());
         if (horizontal())
-          fl_rectf(maximum_position, ty, spacing_, th);
+          draw_filler_( { maximum_position, ty, spacing_, th } );
         else
-          fl_rectf(tx, maximum_position, tw, spacing_);
+          draw_filler_( { tx, maximum_position, tw, spacing_ } );
       }
       if (X != o->x() || Y != o->y() || W != o->w() || H != o->h()) {
         o->resize(X,Y,W,H);
@@ -132,14 +147,12 @@ void Fl_Pack::draw() {
 
   if (horizontal()) {
     if (maximum_position < tx+tw && box()) {
-      fl_color(color());
-      fl_rectf(maximum_position, ty, tx+tw-maximum_position, th);
+      draw_filler_( { maximum_position, ty, tx+tw-maximum_position, th } );
     }
     tw = maximum_position-tx;
   } else {
     if (maximum_position < ty+th && box()) {
-      fl_color(color());
-      fl_rectf(tx, maximum_position, tw, ty+th-maximum_position);
+      draw_filler_( { tx, maximum_position, tw, ty+th-maximum_position } );
     }
     th = maximum_position-ty;
   }
@@ -153,7 +166,13 @@ void Fl_Pack::draw() {
     d = FL_DAMAGE_ALL;
   }
   if (d&FL_DAMAGE_ALL) {
+    if (Fl::box_bg(box())) {
+      // Make sure that we only draw the frame part, because the children
+      // and fillers are already rendered at this point.
+      draw_box(fl_frame(box()), x(), y(), w(), h(), color());
+    } else {
       draw_box();
+    }
     draw_label();
   }
 }

test/preferences.fl

@@ -186,9 +186,11 @@ Fl_Input::paste_menu_text = gettext("Paste");} {}
         MenuItem {} {
           label sandals
           callback {[](Fl_Widget*, void*)
+/* If this code generates an error, you are using an older version of Fluid */
+/* Support for lambda callbacks was added dec 12 2025 */
 {
   puts("The shoe is the sign!");
-}} selected
+}}
           xywh {0 0 100 20}
         }
         MenuItem {} {
@@ -223,6 +225,8 @@ Fl_Input::paste_menu_text = gettext("Paste");} {}
       Fl_Check_Button wShave {
         label shave
         callback {[](Fl_Widget* w, void*)->void {
+/* If this code generates an error, you are using an older version of Fluid */
+/* Support for lambda callbacks was added dec 12 2025 */
   auto* btn = static_cast<Fl_Check_Button*>(w);
   if (btn->value()) {
     puts("Shave.");
@@ -230,7 +234,7 @@ Fl_Input::paste_menu_text = gettext("Paste");} {}
     puts("Don't shave.");
   }
 }}
-        comment {// Testing lambdas for callbacks}
+        comment {// Testing lambdas for callbacks} selected
         xywh {25 212 105 24} down_box DOWN_BOX
       }
       Fl_Check_Button wBrush {