Viewport: Fixed PushClipRectFullScreen() missing out on negative coordinates. Among other things, the outer highlight during CTRL+Tab wouldn't appear in negative coordinates monitors. (~#2176, #1542)

imgui.cpp

@@ -3625,10 +3625,10 @@ void ImGui::NewFrame()
     g.IO.Fonts->Locked = true;
     SetCurrentFont(GetDefaultFont());
     IM_ASSERT(g.Font->IsLoaded());
-    ImVec2 virtual_space_max(0,0);
+    ImRect virtual_space(FLT_MAX, FLT_MAX, -FLT_MAX, -FLT_MAX);
     for (int n = 0; n < g.Viewports.Size; n++)
-        virtual_space_max = ImMax(virtual_space_max, g.Viewports[n]->Pos + g.Viewports[n]->Size);
-    g.DrawListSharedData.ClipRectFullscreen = ImVec4(0.0f, 0.0f, virtual_space_max.x, virtual_space_max.y);
+        virtual_space.Add(g.Viewports[n]->GetRect());
+    g.DrawListSharedData.ClipRectFullscreen = ImVec4(virtual_space.Min.x, virtual_space.Min.y, virtual_space.Max.x, virtual_space.Max.y);
     g.DrawListSharedData.CurveTessellationTol = g.Style.CurveTessellationTol;
 
     // Mark rendering data as invalid to prevent user who may have a handle on it to use it.
@@ -10334,6 +10334,8 @@ ImGuiViewportP* ImGui::AddUpdateViewport(ImGuiWindow* window, ImGuiID id, const
 
         // We normally setup for all viewports in NewFrame() but here need to handle the mid-frame creation of a new viewport.
         // We need to extend the fullscreen clip rect so the OverlayDrawList clip is correct for that the first frame
+        g.DrawListSharedData.ClipRectFullscreen.x = ImMin(g.DrawListSharedData.ClipRectFullscreen.x, viewport->Pos.x);
+        g.DrawListSharedData.ClipRectFullscreen.y = ImMin(g.DrawListSharedData.ClipRectFullscreen.y, viewport->Pos.y);
         g.DrawListSharedData.ClipRectFullscreen.z = ImMax(g.DrawListSharedData.ClipRectFullscreen.z, viewport->Pos.x + viewport->Size.x);
         g.DrawListSharedData.ClipRectFullscreen.w = ImMax(g.DrawListSharedData.ClipRectFullscreen.w, viewport->Pos.y + viewport->Size.y);