diff --git a/imgui.cpp b/imgui.cpp
index f5949408..562b5f93 100644
--- a/imgui.cpp
+++ b/imgui.cpp
@@ -3347,13 +3347,13 @@ static void ImGui::UpdateViewports()
     const ImVec2 mouse_os_pos = ConvertViewportPosToOsDesktopPos(g.IO.MousePos, viewport_ref);
 
     g.CurrentViewport = NULL;
-    for (int n = 1; n < g.Viewports.Size; n++)
+    for (int n = 0; n < g.Viewports.Size; n++)
     {
         // Erase unused viewports
         ImGuiViewport* viewport = g.Viewports[n];
         IM_ASSERT(viewport->Idx == n);
-        IM_ASSERT(!(viewport->Flags & ImGuiViewportFlags_MainViewport));
-        if (viewport->LastFrameActive < g.FrameCount - 2)
+
+        if (n > 0 && viewport->LastFrameActive < g.FrameCount - 2)
         {
             // Translate windows like if we were resizing the viewport to be zero-width
             ResizeViewportTranslateWindows(n + 1, g.Viewports.Size, viewport->Pos.x - viewport->GetNextX(), -1, viewport);
@@ -3380,6 +3380,19 @@ static void ImGui::UpdateViewports()
             if (dx != 0.0f)
                 ResizeViewportTranslateWindows(viewport->Idx + 1, g.Viewports.Size, dx, 0, NULL);
         }
+
+        // Update DPI Scale
+        float new_dpi_scale;
+        if (g.IO.PlatformInterface.GetWindowDpiScale)
+            new_dpi_scale = g.IO.PlatformInterface.GetWindowDpiScale(viewport);
+        else
+            new_dpi_scale = (viewport->PlatformDpiScale != 0.0f) ? viewport->PlatformDpiScale : 1.0f;
+        if (viewport->PlatformDpiScale != 0.0f && new_dpi_scale != viewport->PlatformDpiScale)
+        {
+            float scale_factor = new_dpi_scale / viewport->PlatformDpiScale;
+            ScaleWindowsInViewport(viewport, scale_factor);
+        }
+        viewport->PlatformDpiScale = new_dpi_scale;
     }
 
     // Update main viewport with current size (and OS window position, if known)
@@ -13769,6 +13782,31 @@ static void RenderViewportThumbnail(ImDrawList* draw_list, const ImRect& bb, con
     draw_list->AddRect(bb.Min, bb.Max, ImGui::GetColorU32(ImGuiCol_Border));
 }
 
+static void ScaleWindow(ImGuiWindow* window, float scale)
+{
+    ImVec2 origin = window->Viewport->Pos;
+    window->Pos = window->PosFloat = ImFloor((window->PosFloat - origin) * scale + origin);
+    window->Size = ImFloor(window->Size * scale);
+    window->SizeFull = ImFloor(window->SizeFull * scale);
+    window->SizeContents = ImFloor(window->SizeContents * scale);
+}
+
+// Scale all windows (position, size). Use when e.g. changing DPI. (This is a lossy operation!)
+void ImGui::ScaleWindowsInViewport(ImGuiViewport* viewport, float scale)
+{
+    if (viewport->Window)
+    {
+        ScaleWindow(viewport->Window, scale);
+    }
+    else
+    {
+        ImGuiContext& g = *GImGui;
+        for (int i = 0; i != g.Windows.Size; i++)
+            if (g.Windows[i]->Viewport == viewport)
+                ScaleWindow(g.Windows[i], scale);
+    }
+}
+
 void ImGui::ShowViewportThumbnails()
 {
     ImGuiContext& g = *GImGui;
diff --git a/imgui.h b/imgui.h
index cff33252..529c1a0f 100644
--- a/imgui.h
+++ b/imgui.h
@@ -964,6 +964,7 @@ struct ImGuiPlatformInterface
     void    (*SwapBuffers)(ImGuiViewport* viewport);
 
     // FIXME-DPI
+    float   (*GetWindowDpiScale)(ImGuiViewport* viewport);  // (Optional)
     void    (*BeginViewport)(ImGuiViewport* viewport);      // (Optional) Called during Begin() every time the viewport we are outputting into changes (viewport = next viewport)
     void    (*EndViewport)(ImGuiViewport* viewport);        // (Optional) Called during Begin() every time the viewport we are outputting into changes (viewport = previous viewport)
 };
diff --git a/imgui_internal.h b/imgui_internal.h
index 3dc1b434..17f7c7ef 100644
--- a/imgui_internal.h
+++ b/imgui_internal.h
@@ -546,7 +546,7 @@ struct ImGuiViewport
     bool                PlatformRequestResize;  // Platform window requested resize
     void*               RendererUserData;       // void* to hold custom data structure for the renderer (e.g. framebuffer)
 
-    ImGuiViewport(ImGuiID id, int idx)  { ID = id; Idx = idx; Flags = 0; LastFrameActive = LastFrameAsRefViewport = -1; LastNameHash = 0; Window = NULL; PlatformDpiScale = 1.0f; PlatformUserData = PlatformHandle = NULL; PlatformRequestClose = PlatformRequestResize = false; RendererUserData = NULL; }
+    ImGuiViewport(ImGuiID id, int idx)  { ID = id; Idx = idx; Flags = 0; LastFrameActive = LastFrameAsRefViewport = -1; LastNameHash = 0; Window = NULL; PlatformDpiScale = 0.0f; PlatformUserData = PlatformHandle = NULL; PlatformRequestClose = PlatformRequestResize = false; RendererUserData = NULL; }
     ~ImGuiViewport()                    { IM_ASSERT(PlatformUserData == NULL && RendererUserData == NULL); }
     ImRect  GetRect() const             { return ImRect(Pos.x, Pos.y, Pos.x + Size.x, Pos.y + Size.y); }
     float   GetNextX() const            { const float SPACING = 4.0f; return Pos.x + Size.x + SPACING; }
@@ -1092,6 +1092,7 @@ namespace ImGui
     IMGUI_API ImGuiViewport*        FindViewportByID(ImGuiID id);
     IMGUI_API ImGuiViewport*        FindViewportByPlatformHandle(void* platform_handle);
     IMGUI_API void                  SetNextWindowViewport(ImGuiID id);
+    IMGUI_API void                  ScaleWindowsInViewport(ImGuiViewport* viewport, float scale);
     IMGUI_API void                  ShowViewportThumbnails();
     IMGUI_API void                  DestroyViewportsPlaformData(ImGuiContext* context);
     IMGUI_API void                  DestroyViewportsRendererData(ImGuiContext* context);