Viewport, Platform: Using Platform_GetWindowFocus to provide a much stronger heuristic of platform z-order, in replacement for when the back-end cannot provide io.MouseHoveredViewport. The pressure for it to work well increased with the use of viewports by popups/menus. (#1542)

TODO.txt

@@ -262,7 +262,6 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - viewport: platform: introduce getfocus/setfocus api, so e.g. focus flags can be honored, imgui-side ctrl-tab can focus os window, OS alt-tab can focus imgui window etc.
  - viewport: store per-viewport/monitor DPI in .ini file so an application reload or main window changing DPI on reload can be properly patched for.
  - viewport: vulkan renderer implementation. 
- - viewport: fallback calculation of hovered window is very currently wrong without ImGuiBackendFlags_HasMouseHoveredViewport. typically affect half-overlapping viewported menus. 
  - viewport: need to clarify how to use GetMousePos() from a user point of view.
 
  - inputs: we need an explicit flag about whether the imgui window is focused, to be able to distinguish focused key releases vs alt-tabbing all release behaviors.

imgui.cpp

@@ -3392,8 +3392,8 @@ static void ImGui::UpdateMovingWindow()
 }
 
 // If the back-end doesn't set MouseLastHoveredViewport or doesn't honor ImGuiViewportFlags_NoInputs, we do a search ourselves.
-// This search won't take account of the possibility that non-imgui windows may be in-between our dragged window and our target window.
-// FIXME-VIEWPORT: Need a proper notion of focus. At least use the equivalent of LastFrameAsRefViewport on a per-window basis.
+// A) It won't take account of the possibility that non-imgui windows may be in-between our dragged window and our target window. 
+// B) It requires Platform_GetWindowFocus to be implemented by back-end.
 static ImGuiViewportP* FindViewportHoveredFromPlatformWindowStack(const ImVec2 mouse_platform_pos)
 {
     ImGuiContext& g = *GImGui;
@@ -3402,7 +3402,7 @@ static ImGuiViewportP* FindViewportHoveredFromPlatformWindowStack(const ImVec2 m
     {
         ImGuiViewportP* viewport = g.Viewports[n];
         if (!(viewport->Flags & ImGuiViewportFlags_NoInputs) && viewport->GetRect().Contains(mouse_platform_pos))
-            if (best_candidate == NULL || best_candidate->LastFrameAsRefViewport < viewport->LastFrameAsRefViewport)
+            if (best_candidate == NULL || best_candidate->LastFrontMostStampCount < viewport->LastFrontMostStampCount)
                 best_candidate = viewport;
     }
     return best_candidate;
@@ -3532,10 +3532,8 @@ static void ImGui::UpdateViewports()
     if (!(g.IO.ConfigFlags & ImGuiConfigFlags_ViewportsEnable))
     {
         g.MouseRefViewport = g.MouseRefPrevViewport = main_viewport;
-        g.MouseRefViewport->LastFrameAsRefViewport = g.FrameCount;
         return;
     }
-    g.MouseRefViewport->LastFrameAsRefViewport = g.FrameCount;
 
     // Mouse handling: decide on the actual mouse viewport for this frame between the active/focused viewport and the hovered viewport.
     ImGuiViewportP* viewport_hovered = NULL;
@@ -3584,6 +3582,21 @@ void ImGui::UpdatePlatformWindows()
     if (!(g.IO.ConfigFlags & ImGuiConfigFlags_ViewportsEnable))
         return;
 
+    // Update our implicit z-order knowledge of platform windows, which is used when the back-end cannot provide io.MouseHoveredViewport.
+    if (g.PlatformIO.Platform_GetWindowFocus)
+    {
+        ImGuiViewportP* focused_viewport = NULL;
+        for (int i = 0; i < g.Viewports.Size && focused_viewport == NULL; i++)
+            if (g.PlatformIO.Platform_GetWindowFocus(g.Viewports[i]))
+                focused_viewport = g.Viewports[i];
+        if (focused_viewport && g.PlatformLastFocusedViewport != focused_viewport->ID)
+        {
+            if (focused_viewport->LastFrontMostStampCount != g.WindowsFrontMostStampCount)
+                focused_viewport->LastFrontMostStampCount = ++g.WindowsFrontMostStampCount;
+            g.PlatformLastFocusedViewport = focused_viewport->ID;
+        }
+    }
+
     // Create/resize/destroy platform windows to match each active viewport.
     // Skip the main viewport (index 0), which is always fully handled by the application!
     for (int i = 1; i < g.Viewports.Size; i++)
@@ -3656,7 +3669,7 @@ void ImGui::UpdatePlatformWindows()
             g.PlatformIO.Platform_SetWindowAlpha(viewport, viewport->Alpha);
         viewport->LastAlpha = viewport->Alpha;
 
-        // Show window. On startup ensure platform window don't get focus.
+        // Show window. On startup ensure platform window don't get focus
         if (is_new_window)
         {
             if (g.FrameCount < 2)
@@ -3664,6 +3677,10 @@ void ImGui::UpdatePlatformWindows()
             g.PlatformIO.Platform_ShowWindow(viewport);
         }
 
+        // Even without focus, we assume the window becomes front-most. This is used by our platform z-order heuristic when io.MouseHoveredViewport is not available.
+        if (is_new_window && viewport->LastFrontMostStampCount != g.WindowsFrontMostStampCount)
+            viewport->LastFrontMostStampCount = ++g.WindowsFrontMostStampCount;
+
         // Clear request flags
         viewport->PlatformRequestClose = viewport->PlatformRequestMove = viewport->PlatformRequestResize = false;
     }

imgui.h

@@ -1073,7 +1073,7 @@ struct ImGuiIO
     float       MouseWheel;                     // Mouse wheel Vertical: 1 unit scrolls about 5 lines text. 
     float       MouseWheelH;                    // Mouse wheel Horizontal. Most users don't have a mouse with an horizontal wheel, may not be filled by all back-ends.
     ImGuiID     MousePosViewport;               // (Optional) When using multiple viewports: viewport from which io.MousePos is based from (when dragging this is generally the captured/focused viewport, even though we can drag outside of it and then it's not hovered anymore). (0 == default viewport)
-    ImGuiID     MouseHoveredViewport;           // (Optional) When using multiple viewports: viewport the OS mouse cursor is hovering. (0 == default viewport) _IGNORING_ viewports with the ImGuiBackendFlags_HasMouseHoveredViewport flag, and _REGARDLESS_ of whether another viewport is focused. Set io.BackendFlags |= ImGuiBackendFlags_HasMouseHoveredViewport if you can provide this information.
+    ImGuiID     MouseHoveredViewport;           // (Optional) When using multiple viewports: viewport the OS mouse cursor is hovering _IGNORING_ viewports with the ImGuiViewportFlags_NoInputs flag, and _REGARDLESS_ of whether another viewport is focused. Set io.BackendFlags |= ImGuiBackendFlags_HasMouseHoveredViewport if you can provide this info. If you don't imgui will use a decent heuristic instead.
     bool        MouseDrawCursor;                // Request ImGui to draw a mouse cursor for you (if you are on a platform without a mouse cursor).
     bool        KeyCtrl;                        // Keyboard modifier pressed: Control
     bool        KeyShift;                       // Keyboard modifier pressed: Shift

imgui_internal.h

@@ -511,8 +511,8 @@ struct ImGuiViewportP : public ImGuiViewport
 {
     int                 Idx;
     int                 LastFrameActive;          // Last frame number this viewport was activated by a window
-    int                 LastFrameAsRefViewport;   // Last frame number this viewport was io.MouseViewportRef
     int                 LastFrameOverlayDrawList;
+    int                 LastFrontMostStampCount;  // Last stamp number from when a window hosted by this viewport was made front-most (by comparing this value between two viewport we have an implicit viewport z-order
     ImGuiID             LastNameHash;
     ImVec2              LastPos;
     float               Alpha;                    // Window opacity (when dragging dockable windows/viewports we make them transparent)
@@ -524,7 +524,7 @@ struct ImGuiViewportP : public ImGuiViewport
     ImDrawDataBuilder   DrawDataBuilder;
     ImVec2              RendererLastSize;
 
-    ImGuiViewportP()         { Idx = 1; LastFrameActive = LastFrameAsRefViewport = LastFrameOverlayDrawList = -1; LastNameHash = 0; Alpha = LastAlpha = 1.0f; PlatformMonitor = INT_MIN; Window = NULL; OverlayDrawList = NULL; RendererLastSize = ImVec2(-1.0f,-1.0f); }
+    ImGuiViewportP()         { Idx = 1; LastFrameActive = LastFrameOverlayDrawList = LastFrontMostStampCount = -1; LastNameHash = 0; Alpha = LastAlpha = 1.0f; PlatformMonitor = INT_MIN; Window = NULL; OverlayDrawList = NULL; RendererLastSize = ImVec2(-1.0f,-1.0f); }
     ~ImGuiViewportP()        { if (OverlayDrawList) IM_DELETE(OverlayDrawList); }
     ImRect  GetRect() const  { return ImRect(Pos.x, Pos.y, Pos.x + Size.x, Pos.y + Size.y); }
     ImVec2  GetCenter() const{ return ImVec2(Pos.x + Size.x * 0.5f, Pos.y + Size.y * 0.5f); }
@@ -610,6 +610,7 @@ struct ImGuiContext
     ImVector<ImGuiWindow*>  CurrentWindowStack;
     ImGuiStorage            WindowsById;
     int                     WindowsActiveCount;
+    int                     WindowsFrontMostStampCount;         // Every time the front-most window changes, we stamp its viewport with an incrementing counter
     ImGuiWindow*            CurrentWindow;                      // Being drawn into
     ImGuiWindow*            HoveredWindow;                      // Will catch mouse inputs
     ImGuiWindow*            HoveredRootWindow;                  // Will catch mouse inputs (for focus/move only)
@@ -644,6 +645,7 @@ struct ImGuiContext
     ImGuiViewportP*         MouseRefPrevViewport;
     ImGuiViewportP*         MouseHoveredLastViewport;           // Last viewport that was hovered by mouse (even if we are not hovering any viewport any more)
     ImGuiID                 MouseClickedPosViewportId[5];       // For rarely used fields we only compare to, store viewport ID only so we don't have to clean dangling pointers
+    ImGuiID                 PlatformLastFocusedViewport;        // Record of last focused platform window/viewport, when this changes we stamp the viewport as front-most
 
     // Navigation data (for gamepad/keyboard)
     ImGuiWindow*            NavWindow;                          // Focused window for navigation. Could be called 'FocusWindow'
@@ -751,6 +753,7 @@ struct ImGuiContext
         FrameCount = 0;
         FrameCountEnded = FrameCountPlatformEnded = FrameCountRendered = -1;
         WindowsActiveCount = 0;
+        WindowsFrontMostStampCount = 0;
         CurrentWindow = NULL;
         HoveredWindow = NULL;
         HoveredRootWindow = NULL;
@@ -776,6 +779,7 @@ struct ImGuiContext
         MouseRefViewport = NULL;
         MouseRefPrevViewport = MouseHoveredLastViewport = NULL;
         memset(MouseClickedPosViewportId, 0, sizeof(MouseClickedPosViewportId));
+        PlatformLastFocusedViewport = 0;
 
         NavWindow = NULL;
         NavId = NavActivateId = NavActivateDownId = NavActivatePressedId = NavInputId = 0;