Viewports: moved Platform_GetWindowFocus queries in UpdateViewportsNewFrame(). Added ImGuiViewportFlags_IsFocused status flag. (#1542)

Not sure why queries were in UpdatePlatformWindows().
- initially added there on 2018/04/26 f1ae07e5321014deaa9920b89e72b1faf9a95700 (squashed)
- slightly moved in cd51f37fc04eb3b260940f7900afb84860119fc5 for the purpose of putting less constraint on backend but that check is now done on our side anyhow.
Seems more consistent to do it nxt to other polling in UpdateViewportsNewFrame().
Not using ImGuiViewportFlags_IsFocused yet.

imgui.cpp

@@ -13874,21 +13874,54 @@ static void ImGui::UpdateViewportsNewFrame()
     IM_ASSERT(g.PlatformIO.Viewports.Size <= g.Viewports.Size);
 
     // Update Minimized status (we need it first in order to decide if we'll apply Pos/Size of the main viewport)
+    // Update Focused status
     const bool viewports_enabled = (g.ConfigFlagsCurrFrame & ImGuiConfigFlags_ViewportsEnable) != 0;
     if (viewports_enabled)
     {
+        ImGuiViewportP* focused_viewport = NULL;
         for (int n = 0; n < g.Viewports.Size; n++)
         {
             ImGuiViewportP* viewport = g.Viewports[n];
             const bool platform_funcs_available = viewport->PlatformWindowCreated;
             if (g.PlatformIO.Platform_GetWindowMinimized && platform_funcs_available)
             {
-                bool minimized = g.PlatformIO.Platform_GetWindowMinimized(viewport);
-                if (minimized)
+                bool is_minimized = g.PlatformIO.Platform_GetWindowMinimized(viewport);
+                if (is_minimized)
                     viewport->Flags |= ImGuiViewportFlags_IsMinimized;
                 else
                     viewport->Flags &= ~ImGuiViewportFlags_IsMinimized;
             }
+
+            // Update our implicit z-order knowledge of platform windows, which is used when the backend cannot provide io.MouseHoveredViewport.
+            // When setting Platform_GetWindowFocus, it is expected that the platform backend can handle calls without crashing if it doesn't have data stored.
+            if (g.PlatformIO.Platform_GetWindowFocus && platform_funcs_available)
+            {
+                bool is_focused = g.PlatformIO.Platform_GetWindowFocus(viewport);
+                if (is_focused)
+                    viewport->Flags |= ImGuiViewportFlags_IsFocused;
+                else
+                    viewport->Flags &= ~ImGuiViewportFlags_IsFocused;
+                if (is_focused)
+                    focused_viewport = viewport;
+            }
+        }
+
+        // Focused viewport has changed?
+        if (focused_viewport && g.PlatformLastFocusedViewportId != focused_viewport->ID)
+        {
+            // Store a tag so we can infer z-order easily from all our windows
+            // We compare PlatformLastFocusedViewportId so newly created viewports with _NoFocusOnAppearing flag
+            // will keep the front most stamp instead of losing it back to their parent viewport.
+            if (focused_viewport->LastFocusedStampCount != g.ViewportFocusedStampCount)
+                focused_viewport->LastFocusedStampCount = ++g.ViewportFocusedStampCount;
+            g.PlatformLastFocusedViewportId = focused_viewport->ID;
+
+            // Focus associated dear imgui window (#6299)
+            // FIXME: perhaps 'FocusTopMostWindowUnderOne()' can handle both cases?
+            if (focused_viewport->Window != NULL)
+                FocusWindow(NavRestoreLastChildNavWindow(focused_viewport->Window));
+            else
+                FocusTopMostWindowUnderOne(NULL, NULL, focused_viewport);
         }
     }
 
@@ -14088,7 +14121,7 @@ ImGuiViewportP* ImGui::AddUpdateViewport(ImGuiWindow* window, ImGuiID id, const
             viewport->Pos = pos;
         if (!viewport->PlatformRequestResize || viewport->ID == IMGUI_VIEWPORT_DEFAULT_ID)
             viewport->Size = size;
-        viewport->Flags = flags | (viewport->Flags & ImGuiViewportFlags_IsMinimized); // Preserve existing flags
+        viewport->Flags = flags | (viewport->Flags & (ImGuiViewportFlags_IsMinimized | ImGuiViewportFlags_IsFocused)); // Preserve existing flags
     }
     else
     {
@@ -14472,38 +14505,6 @@ void ImGui::UpdatePlatformWindows()
         // Clear request flags
         viewport->ClearRequestFlags();
     }
-
-    // Update our implicit z-order knowledge of platform windows, which is used when the backend cannot provide io.MouseHoveredViewport.
-    // When setting Platform_GetWindowFocus, it is expected that the platform backend can handle calls without crashing if it doesn't have data stored.
-    // FIXME-VIEWPORT: We should use this information to also set dear imgui-side focus, allowing us to handle os-level alt+tab.
-    if (g.PlatformIO.Platform_GetWindowFocus != NULL)
-    {
-        ImGuiViewportP* focused_viewport = NULL;
-        for (int n = 0; n < g.Viewports.Size && focused_viewport == NULL; n++)
-        {
-            ImGuiViewportP* viewport = g.Viewports[n];
-            if (viewport->PlatformWindowCreated)
-                if (g.PlatformIO.Platform_GetWindowFocus(viewport))
-                    focused_viewport = viewport;
-        }
-
-        // Focused viewport has changed?
-        if (focused_viewport && g.PlatformLastFocusedViewportId != focused_viewport->ID)
-        {
-            // Store a tag so we can infer z-order easily from all our windows
-            // We compare PlatformLastFocusedViewportId so newly created viewports with _NoFocusOnAppearing flag
-            // will keep the front most stamp instead of losing it back to their parent viewport.
-            if (focused_viewport->LastFocusedStampCount != g.ViewportFocusedStampCount)
-                focused_viewport->LastFocusedStampCount = ++g.ViewportFocusedStampCount;
-            g.PlatformLastFocusedViewportId = focused_viewport->ID;
-
-            // Focus associated dear imgui window (#6299)
-            if (focused_viewport->Window != NULL)
-                FocusWindow(NavRestoreLastChildNavWindow(focused_viewport->Window));
-            else
-                FocusTopMostWindowUnderOne(NULL, NULL, focused_viewport);
-        }
-    }
 }
 
 // This is a default/basic function for performing the rendering/swap of multiple Platform Windows.
@@ -19915,9 +19916,11 @@ void ImGui::DebugNodeViewport(ImGuiViewportP* viewport)
             viewport->WorkOffsetMin.x, viewport->WorkOffsetMin.y, viewport->WorkOffsetMax.x, viewport->WorkOffsetMax.y,
             viewport->PlatformMonitor, viewport->DpiScale * 100.0f);
         if (viewport->Idx > 0) { SameLine(); if (SmallButton("Reset Pos")) { viewport->Pos = ImVec2(200, 200); viewport->UpdateWorkRect(); if (viewport->Window) viewport->Window->Pos = viewport->Pos; } }
-        BulletText("Flags: 0x%04X =%s%s%s%s%s%s%s%s%s%s%s%s", viewport->Flags,
+        BulletText("Flags: 0x%04X =%s%s%s%s%s%s%s%s%s%s%s%s%s", viewport->Flags,
             //(flags & ImGuiViewportFlags_IsPlatformWindow) ? " IsPlatformWindow" : "", // Omitting because it is the standard
             (flags & ImGuiViewportFlags_IsPlatformMonitor) ? " IsPlatformMonitor" : "",
+            (flags & ImGuiViewportFlags_IsMinimized) ? " IsMinimized" : "",
+            (flags & ImGuiViewportFlags_IsFocused) ? " IsFocused" : "",
             (flags & ImGuiViewportFlags_OwnedByApp) ? " OwnedByApp" : "",
             (flags & ImGuiViewportFlags_NoDecoration) ? " NoDecoration" : "",
             (flags & ImGuiViewportFlags_NoTaskBarIcon) ? " NoTaskBarIcon" : "",
@@ -19925,9 +19928,8 @@ void ImGui::DebugNodeViewport(ImGuiViewportP* viewport)
             (flags & ImGuiViewportFlags_NoFocusOnClick) ? " NoFocusOnClick" : "",
             (flags & ImGuiViewportFlags_NoInputs) ? " NoInputs" : "",
             (flags & ImGuiViewportFlags_NoRendererClear) ? " NoRendererClear" : "",
-            (flags & ImGuiViewportFlags_TopMost) ? " TopMost" : "",
-            (flags & ImGuiViewportFlags_IsMinimized) ? " IsMinimized" : "",
             (flags & ImGuiViewportFlags_NoAutoMerge) ? " NoAutoMerge" : "",
+            (flags & ImGuiViewportFlags_TopMost) ? " TopMost" : "",
             (flags & ImGuiViewportFlags_CanHostOtherWindows) ? " CanHostOtherWindows" : "");
         for (int layer_i = 0; layer_i < IM_ARRAYSIZE(viewport->DrawDataBuilder.Layers); layer_i++)
             for (int draw_list_i = 0; draw_list_i < viewport->DrawDataBuilder.Layers[layer_i].Size; draw_list_i++)

imgui.h

@@ -3088,6 +3088,7 @@ enum ImGuiViewportFlags_
 
     // Output status flags (from Platform)
     ImGuiViewportFlags_IsMinimized              = 1 << 12,  // Platform Window: Window is minimized, can skip render. When minimized we tend to avoid using the viewport pos/size for clipping window or testing if they are contained in the viewport.
+    ImGuiViewportFlags_IsFocused                = 1 << 13,  // Platform Window: Window is focused (last call to Platform_GetWindowFocus() returned true)
 };
 
 // - Currently represents the Platform Window created by the application which is hosting our Dear ImGui windows.