There is a bug in the macOS window creation that misplaces windows if they are shown whilst in fullscreen mode. This bug has probably been around for ages, we just haven't noticed it until know as our other windows were previously all dialogs (which don't trigger the cascading offset for some reason).

I've confirmed that this fixes the issue:

diff --git a/src/Fl_cocoa.mm b/src/Fl_cocoa.mm
index be07b09fb..341d91a3c 100644
--- a/src/Fl_cocoa.mm
+++ b/src/Fl_cocoa.mm
@@ -3299,7 +3299,7 @@ void Fl_X::make(Fl_Window* w)
     [cw setLevel:winlevel];
     
     q_set_window_title(cw, w->label(), w->iconlabel());
-    if (!w->force_position()) {
+    if (!w->force_position() && !w->fullscreen_active()) {
       if (w->modal()) {
         [cw center];
       } else if (w->non_modal()) {

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