Viewports, Internals: added GetViewportPlatformMonitor() will a safety net to keep code portable + simplified handling of disconnected monitor in Begin().

imgui.cpp

@@ -6449,21 +6449,13 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
             }
             else if (window->ViewportOwned && g.PlatformIO.Monitors.Size > 0)
             {
-                if (window->Viewport->PlatformMonitor == -1)
-                {
-                    // Fallback for "lost" window (e.g. a monitor disconnected): we move the window back over the main viewport
-                    const ImGuiViewport* main_viewport = GetMainViewport();
-                    SetWindowPos(window, main_viewport->Pos + style.DisplayWindowPadding, ImGuiCond_Always);
-                }
-                else
-                {
-                    ImGuiPlatformMonitor& monitor = g.PlatformIO.Monitors[window->Viewport->PlatformMonitor];
-                    visibility_rect.Min = monitor.WorkPos + visibility_padding;
-                    visibility_rect.Max = monitor.WorkPos + monitor.WorkSize - visibility_padding;
+                // Lost windows (e.g. a monitor disconnected) will naturally moved to the fallback/dummy monitor aka the main viewport.
+                const ImGuiPlatformMonitor* monitor = GetViewportPlatformMonitor(window->Viewport);
+                visibility_rect.Min = monitor->WorkPos + visibility_padding;
+                visibility_rect.Max = monitor->WorkPos + monitor->WorkSize - visibility_padding;
                 ClampWindowRect(window, visibility_rect);
             }
         }
-        }
         window->Pos = ImFloor(window->Pos);
 
         // Lock window rounding for the frame (so that altering them doesn't cause inconsistencies)
@@ -11534,6 +11526,17 @@ static void ImGui::UpdateViewportsNewFrame()
         viewport->DpiScale = new_dpi_scale;
     }
 
+    // Update fallback monitor
+    if (g.PlatformIO.Monitors.Size == 0)
+    {
+        ImGuiPlatformMonitor* monitor = &g.FallbackMonitor;
+        monitor->MainPos = main_viewport->Pos;
+        monitor->MainSize = main_viewport->Size;
+        monitor->WorkPos = main_viewport->WorkPos;
+        monitor->WorkSize = main_viewport->WorkSize;
+        monitor->DpiScale = main_viewport->DpiScale;
+    }
+
     if (!viewports_enabled)
     {
         g.MouseViewport = main_viewport;
@@ -12010,6 +12013,17 @@ static void ImGui::UpdateViewportPlatformMonitor(ImGuiViewportP* viewport)
     viewport->PlatformMonitor = (short)FindPlatformMonitorForRect(viewport->GetMainRect());
 }
 
+// Return value is always != NULL, but don't hold on it across frames.
+const ImGuiPlatformMonitor* ImGui::GetViewportPlatformMonitor(ImGuiViewport* viewport_p)
+{
+    ImGuiContext& g = *GImGui;
+    ImGuiViewportP* viewport = (ImGuiViewportP*)(void*)viewport_p;
+    int monitor_idx = viewport->PlatformMonitor;
+    if (monitor_idx >= 0 || monitor_idx < g.PlatformIO.Monitors.Size)
+        return &g.PlatformIO.Monitors[monitor_idx];
+    return &g.FallbackMonitor;
+}
+
 void ImGui::DestroyPlatformWindow(ImGuiViewportP* viewport)
 {
     ImGuiContext& g = *GImGui;

imgui_internal.h

@@ -1542,6 +1542,7 @@ struct ImGuiContext
     ImGuiViewportP*         MouseViewport;
     ImGuiViewportP*         MouseLastHoveredViewport;           // Last known viewport that was hovered by mouse (even if we are not hovering any viewport any more) + honoring the _NoInputs flag.
     ImGuiID                 PlatformLastFocusedViewportId;
+    ImGuiPlatformMonitor    FallbackMonitor;                    // Virtual monitor used as fallback if backend doesn't provide monitor information.
     int                     ViewportFrontMostStampCount;        // Every time the front-most window changes, we stamp its viewport with an incrementing counter
 
     // Gamepad/keyboard Navigation
@@ -2455,6 +2456,7 @@ namespace ImGui
     IMGUI_API void                  TranslateWindowsInViewport(ImGuiViewportP* viewport, const ImVec2& old_pos, const ImVec2& new_pos);
     IMGUI_API void                  ScaleWindowsInViewport(ImGuiViewportP* viewport, float scale);
     IMGUI_API void                  DestroyPlatformWindow(ImGuiViewportP* viewport);
+    const ImGuiPlatformMonitor*     GetViewportPlatformMonitor(ImGuiViewport* viewport);
 
     // Settings
     IMGUI_API void                  MarkIniSettingsDirty();