Fl_Tree: make automatic resizing of children optional (STR 3030)

- add getter and setter methods:
  - int Fl_Tree::auto_resize_children()
  - void Fl_Tree::auto_resize_children(int mode)

Note: the default in 1.5.0 is "OFF", i.e. no automatic resizing.
  The methods above can be used to switch it on if required.

This is a behavioral change to fix a bug in 1.4 and lower. It will not
be backported because it would break the ABI.

FL/Fl_Tree.H

@@ -1,4 +1,19 @@
 //
+// Fl_Tree.H -- This file is part of the Fl_Tree widget for FLTK
+//
+// Copyright 2009-2010 by Greg Ercolano.
+// Copyright 2011-2026 by Bill Spitzak and others.
+//
+// This library is free software. Distribution and use rights are outlined in
+// the file "COPYING" which should have been included with this file.  If this
+// file is missing or damaged, see the license at:
+//
+//     https://www.fltk.org/COPYING.php
+//
+// Please see the following page on how to report bugs and issues:
+//
+//     https://www.fltk.org/bugs.php
+//
 
 #ifndef FL_TREE_H
 #define FL_TREE_H
@@ -11,24 +26,6 @@
 #include <FL/Fl_Tree_Item.H>
 #include <FL/Fl_Tree_Prefs.H>
 
-//////////////////////
-// FL/Fl_Tree.H
-//////////////////////
-//
-// Fl_Tree -- This file is part of the Fl_Tree widget for FLTK
-// Copyright (C) 2009-2010 by Greg Ercolano.
-//
-// This library is free software. Distribution and use rights are outlined in
-// the file "COPYING" which should have been included with this file.  If this
-// file is missing or damaged, see the license at:
-//
-//     https://www.fltk.org/COPYING.php
-//
-// Please see the following page on how to report bugs and issues:
-//
-//     https://www.fltk.org/bugs.php
-//
-
 ///
 /// \file
 /// \brief This file contains the definitions of the Fl_Tree class
@@ -293,7 +290,9 @@ class FL_EXPORT Fl_Tree : public Fl_Group {
   int            _scrollbar_size;               // size of scrollbar trough
   Fl_Tree_Item  *_lastselect;                   // last selected item
   char           _lastpushed;                   // FL_PUSH occurred on: 0=nothing, 1=open/close, 2=usericon, 3=label
-  void fix_scrollbar_order();
+  int            _auto_resize_children;         // if true: resize children when the Fl_Tree container is resized
+
+  void           fix_scrollbar_order();         // internal: rearrange scrollbars in list of children
 
 protected:
   Fl_Scrollbar *_vscroll;       ///< Vertical scrollbar
@@ -321,6 +320,58 @@ public:
   void show_self();
   void resize(int,int,int,int) override;
 
+  /**
+    Set mode to resize children when the Fl_Tree container widget is resized.
+
+    The default value is \c false, meaning that child widgets within Fl_Tree
+    widgets are not automatically resized.
+
+    This default behavior was changed in version 1.5.0 because resizing children
+    automatically led to unpredictable behavior depending on their positions and
+    sizes. This was caused by using the default resizing mechanism, Fl_Group::resize(),
+    and was fixed in version 1.5.0 by disabling automatic resizing of children.
+
+    This method can be used to restore the 1.4.x behavior to resize children
+    automatically for backwards compatibility by calling
+    Fl_Tree::auto_resize_children(1). See example below.
+
+    \b Note: use with care and only if required for strict backwards compatibility.
+        The resulting resize behavior can be \e surprising. If your development
+        target is FLTK 1.4 or lower you can't use this method (you may include
+        it inside conditional compilation for FLTK 1.5 and higher).
+
+    Example code for strict backwards compatibility to 1.4.x or lower:
+
+    \code
+    #include <FL/Fl_Tree.H>
+      Fl_Tree *tree = new Fl_Tree(0, 0, 400, 400);
+    #if FL_API_VERSION >= 10500
+      tree->auto_resize_children(1);
+    #endif
+    \endcode
+
+    \param[in]  mode  enable auto resizing if true (non-zero)
+
+    \since 1.5.0
+    \see int auto_resize_children()
+  */
+  void auto_resize_children(int mode) {
+    _auto_resize_children = (mode) ? 1 : 0;
+  }
+
+  /**
+    Returns whether auto resizing of children is active.
+
+    \returns  0 (auto resizing \b off) or 1 ( \b on)
+
+    \since 1.5.0
+
+    \see void auto_resize_children(int mode) for details
+  */
+  int auto_resize_children() {
+    return _auto_resize_children;
+  }
+
   ///////////////////////
   // root methods
   ///////////////////////

src/Fl_Tree.cxx

@@ -1,19 +1,8 @@
 //
-
+// Fl_Tree.cxx -- This file is part of the Fl_Tree widget for FLTK
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
-
-#include <FL/Fl_Tree.H>
-#include <FL/Fl_Preferences.H>
-#include <FL/fl_string_functions.h>
-
-//////////////////////
-// Fl_Tree.cxx
-//////////////////////
 //
-// Fl_Tree -- This file is part of the Fl_Tree widget for FLTK
+// Copyright 2009-2010 by Greg Ercolano.
-// Copyright (C) 2009-2010 by Greg Ercolano.
+// Copyright 2011-2026 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -26,6 +15,14 @@
 //     https://www.fltk.org/bugs.php
 //
 
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <FL/Fl_Tree.H>
+#include <FL/Fl_Preferences.H>
+#include <FL/fl_string_functions.h>
+
 // INTERNAL: scroller callback (hor+vert scroll)
 static void scroll_cb(Fl_Widget*,void *data) {
   ((Fl_Tree*)data)->redraw();
@@ -79,12 +76,13 @@ Fl_Tree::Fl_Tree(int X, int Y, int W, int H, const char *L) : Fl_Group(X,Y,W,H,L
   _root = new Fl_Tree_Item(this);
   _root->parent(0);                             // we are root of tree
   _root->label("ROOT");
-  _item_focus      = 0;
+  _item_focus           = 0;
-  _callback_item   = 0;
+  _callback_item        = 0;
-  _callback_reason = FL_TREE_REASON_NONE;
+  _callback_reason      = FL_TREE_REASON_NONE;
-  _scrollbar_size  = 0;                         // 0: uses Fl::scrollbar_size()
+  _scrollbar_size       = 0;                    // 0: uses Fl::scrollbar_size()
-
+  _lastselect           = nullptr;
-  _lastselect       = 0;
+  _lastpushed           = 0;
+  _auto_resize_children = 0;                    // don't resize children automatically
 
   box(FL_DOWN_BOX);
   color(FL_BACKGROUND2_COLOR, FL_SELECTION_COLOR);
@@ -691,9 +689,13 @@ void Fl_Tree::calc_tree() {
 
 void Fl_Tree::resize(int X,int Y,int W, int H) {
   fix_scrollbar_order();
-  Fl_Group::resize(X,Y,W,H);
+  if (auto_resize_children()) {         // backwards compatibility to 1.4.x
+    Fl_Group::resize(X, Y, W, H);
+    init_sizes();                       // remove stored sizes
+  } else {
+    Fl_Widget::resize(X, Y, W, H);
+  }
   calc_dimensions();
-  init_sizes();
 }
 
 /// Standard FLTK draw() method, handles drawing the tree widget.