Debugging things in VS it seems the problem lies in fltk.lib in the gdi driver code:

     fluid.exe!operator delete(void * block) Line 38    C++
     fluid.exe!operator delete[](void * block) Line 32    C++
>    fluid.exe!Fl_GDI_Graphics_Driver::create_bitmask(int w, int h, const unsigned char * data) Line 393    C++
     fluid.exe!Fl_GDI_Graphics_Driver::cache(Fl_Pixmap * img) Line 806    C++
     fluid.exe!Fl_Graphics_Driver::draw_pixmap(Fl_Pixmap * pxm, int XP, int YP, int WP, int HP, int cx, int cy) Line 242    C++
     fluid.exe!Fl_Pixmap::draw(int XP, int YP, int WP, int HP, int cx, int cy) Line 48    C++
     fluid.exe!Fl_Image::draw(int X, int Y) Line 295    C++
     fluid.exe!Fl_Tree_Item::draw(int X, int & Y, int W, Fl_Tree_Item * itemfocus, int & tree_item_xmax, int lastchild, int render) Line 1104    C++
     fluid.exe!Fl_Tree::draw() Line 733    C++
     fluid.exe!Fl_Group::draw_child(Fl_Widget & widget) Line 961    C++
     fluid.exe!Fl_Group::draw_children() Line 917    C++
     fluid.exe!Fl_Group::draw() Line 932    C++
     fluid.exe!Fl_Group::draw_child(Fl_Widget & widget) Line 961    C++
     fluid.exe!Fl_Group::draw_children() Line 917    C++
     fluid.exe!Fl_Window::draw() Line 523    C++
     fluid.exe!Overlay_Window::draw() Line 344    C++
     fluid.exe!Fl_Window_Driver::draw() Line 82    C++
     fluid.exe!Fl_WinAPI_Window_Driver::flush_overlay() Line 349    C++
     fluid.exe!Fl_Overlay_Window::flush() Line 53    C++
     fluid.exe!Fl_Window_Driver::flush() Line 204    C++
     fluid.exe!Fl::flush() Line 755    C++
     fluid.exe!Fl_WinAPI_System_Driver::wait(double time_to_wait) Line 521    C++
     fluid.exe!Fl::wait(double time_to_wait) Line 581    C++
     fluid.exe!Fl::run() Line 600    C++
     fluid.exe!main(int argc, char * * argv) Line 2023    C++
     fluid.exe!WinMain(HINSTANCE__ * hInstance, HINSTANCE__ * hPrevInstance, char * lpCmdLine, int nCmdShow) Line 113    C

On Thursday, November 24, 2022 at 2:43:34 PM UTC+3 Mo_Al_ wrote:

Thanks for checking things out.

The second issue I only noticed on Windows. This is a backtrace using windbg (since I don't think valgrind runs on windows):
HEAP[fluid.exe]: Heap block at 00000144B3F1D290 modified at 00000144B3F1D2E0 past requested size of 40 (52d8.224c): Break instruction exception - code 80000003 (first chance) ntdll!RtlpBreakPointHeap+0x16: 00007ff9`ba2a70ce cc int 3

I'll try to build a debug build of fluid and run again via windbg.

On Thursday, November 24, 2022 at 1:43:21 PM UTC+3 Matthias Melcher wrote:

may64...@gmail.com schrieb am Donnerstag, 24. November 2022 um 09:17:21 UTC+1:

- The orientation changes (HORIZONTAL -> VERTICAL) when adding widgets into the Fl_Flex:

Ooopsie. I bailed out of the function when there were no children without actually changing the type. Fixed. 

- I had a spurious crash on windows when adding widgets into the Fl_Flex:

I tried to trigger this under Valgrind, but did not find anything. If you have anything repeatable, that would be really helpful.

I did fix two other Valgrind hits though: git commit 6f36a6362982b08574e27a61d9c8716fe5aac8a8