Internals: Window: Store whether the windows has a close button (we need that info for window/tabs dropping preview calculation)

imgui.cpp

@@ -1800,6 +1800,7 @@ ImGuiWindow::ImGuiWindow(const char* name)
     Collapsed = false;
     SkipItems = false;
     Appearing = false;
+    CloseButton = false;
     BeginCount = 0;
     PopupId = 0;
     AutoFitFramesX = AutoFitFramesY = -1;
@@ -4130,6 +4131,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
 
     const bool window_just_appearing_after_hidden_for_resize = (window->HiddenFrames == 1);
     window->Appearing = (window_just_activated_by_user || window_just_appearing_after_hidden_for_resize);
+    window->CloseButton = (p_open != NULL);
 
     // Process SetNextWindow***() calls
     bool window_pos_set_by_api = false, window_size_set_by_api = false;

imgui_internal.h

@@ -700,6 +700,7 @@ struct IMGUI_API ImGuiWindow
     bool                    Collapsed;                          // Set when collapsing window to become only title-bar
     bool                    SkipItems;                          // Set when items can safely be all clipped (e.g. window not visible or collapsed)
     bool                    Appearing;                          // Set during the frame where the window is appearing (or re-appearing)
+    bool                    CloseButton;                        // Set when the window has a close button (p_open != NULL)
     int                     BeginCount;                         // Number of Begin() during the current frame (generally 0 or 1, 1+ if appending via multiple Begin/End pairs)
     ImGuiID                 PopupId;                            // ID in the popup stack when this window is used as a popup/menu (because we use generic Name/ID for recycling)
     int                     AutoFitFramesX, AutoFitFramesY;