Viewports, Docking: Added per-viewport work area system for e.g. menu-bars. Fixed DocksapceOverViewport() and demo code (overlay etc) (ocornut#3035, ocornut#2889, ocornut#2474, ocornut#1542, ocornut#2109)

Clarified that BeginMenuMainBar() had an incorrect knowledge of its height (which was previously harmless).
Designed to easily allow for status bars although we don't have/use them yet, but custom code could use them.

Author: ocornut

imgui.cpp

@@ -5273,6 +5273,7 @@ static ImVec2 CalcWindowAutoFitSize(ImGuiWindow* window, const ImVec2& size_cont
         if (is_popup || is_menu) // Popups and menus bypass style.WindowMinSize by default, but we give then a non-zero minimum size to facilitate understanding problematic cases (e.g. empty popups)
             size_min = ImMin(size_min, ImVec2(4.0f, 4.0f));
 
+        // FIXME-VIEWPORT-WORKAREA: May want to use GetWorkSize() instead of Size depending on the type of windows?
         ImVec2 avail_size = window->Viewport->Size;
         if (window->ViewportOwned)
             avail_size = ImVec2(FLT_MAX, FLT_MAX);
@@ -6002,6 +6003,10 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         else
             window->WindowPadding = style.WindowPadding;
 
+        // Lock menu offset so size calculation can use it as menu-bar windows need a minimum size.
+        window->DC.MenuBarOffset.x = ImMax(ImMax(window->WindowPadding.x, style.ItemSpacing.x), g.NextWindowData.MenuBarOffsetMinVal.x);
+        window->DC.MenuBarOffset.y = g.NextWindowData.MenuBarOffsetMinVal.y;
+
         // Collapse window by double-clicking on title bar
         // At this point we don't have a clipping rectangle setup yet, so we can use the title bar area for hit detection and drawing
         if (!(flags & ImGuiWindowFlags_NoTitleBar) && !(flags & ImGuiWindowFlags_NoCollapse) && !window->DockIsActive)
@@ -6182,13 +6187,14 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
 
         // Clamp position/size so window stays visible within its viewport or monitor
         // Ignore zero-sized display explicitly to avoid losing positions if a window manager reports zero-sized window when initializing or minimizing.
+        // FIXME: Similar to code in GetWindowAllowedExtentRect()
         ImRect viewport_rect = window->Viewport->GetMainRect();
         if (!window_pos_set_by_api && !(flags & ImGuiWindowFlags_ChildWindow) && window->AutoFitFramesX <= 0 && window->AutoFitFramesY <= 0)
         {
             ImVec2 clamp_padding = ImMax(style.DisplayWindowPadding, style.DisplaySafeAreaPadding);
             if (!window->ViewportOwned && viewport_rect.GetWidth() > 0 && viewport_rect.GetHeight() > 0.0f)
             {
-                ClampWindowRect(window, viewport_rect, clamp_padding);
+                ClampWindowRect(window, window->Viewport->GetWorkRect(), clamp_padding);
             }
             else if (window->ViewportOwned && g.PlatformIO.Monitors.Size > 0)
             {
@@ -6437,8 +6443,6 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         window->DC.NavHasScroll = (window->ScrollMax.y > 0.0f);
 
         window->DC.MenuBarAppending = false;
-        window->DC.MenuBarOffset.x = ImMax(ImMax(window->WindowPadding.x, style.ItemSpacing.x), g.NextWindowData.MenuBarOffsetMinVal.x);
-        window->DC.MenuBarOffset.y = g.NextWindowData.MenuBarOffsetMinVal.y;
         window->DC.MenuColumns.Update(3, style.ItemSpacing.x, window_just_activated_by_user);
         window->DC.TreeDepth = 0;
         window->DC.TreeJumpToParentOnPopMask = 0x00;
@@ -8514,6 +8518,7 @@ ImVec2 ImGui::FindBestWindowPosForPopupEx(const ImVec2& ref_pos, const ImVec2& s
     return pos;
 }
 
+// Note that this is used for popups, which can overlap the non work-area of individual viewports.
 ImRect ImGui::GetWindowAllowedExtentRect(ImGuiWindow* window)
 {
     ImGuiContext& g = *GImGui;
@@ -8527,6 +8532,7 @@ ImRect ImGui::GetWindowAllowedExtentRect(ImGuiWindow* window)
     }
     else
     {
+        // Use the full viewport area (not work area) for popups
         r_screen.Min = window->Viewport->Pos;
         r_screen.Max = window->Viewport->Pos + window->Viewport->Size;
     }
@@ -10731,6 +10737,11 @@ static void ImGui::UpdateViewportsNewFrame()
         // Update/copy monitor info
         UpdateViewportPlatformMonitor(viewport);
 
+        // Lock down space taken by menu bars and status bars, reset the offset for fucntions like BeginMainMenuBar() to alter them again.
+        viewport->WorkOffsetMin = viewport->CurrWorkOffsetMin;
+        viewport->WorkOffsetMax = viewport->CurrWorkOffsetMax;
+        viewport->CurrWorkOffsetMin = viewport->CurrWorkOffsetMax = ImVec2(0.0f, 0.0f);
+
         // Reset alpha every frame. Users of transparency (docking) needs to request a lower alpha back.
         viewport->Alpha = 1.0f;
 
@@ -13852,14 +13863,14 @@ void ImGui::DockSpace(ImGuiID id, const ImVec2& size_arg, ImGuiDockNodeFlags fla
 // Tips: Use with ImGuiDockNodeFlags_PassthruCentralNode!
 // The limitation with this call is that your window won't have a menu bar.
 // Even though we could pass window flags, it would also require the user to be able to call BeginMenuBar() somehow meaning we can't Begin/End in a single function.
-// So if you want a menu bar you need to repeat this code manually ourselves. As with advanced other Docking API, we may change this function signature.
+// But you can also use BeginMainMenuBar(). If you really want a menu bar inside the same window as the one hosting the dockspace, you will need to copy this code somewhere and tweak it.
 ImGuiID ImGui::DockSpaceOverViewport(ImGuiViewport* viewport, ImGuiDockNodeFlags dockspace_flags, const ImGuiWindowClass* window_class)
 {
     if (viewport == NULL)
         viewport = GetMainViewport();
 
-    SetNextWindowPos(viewport->Pos);
-    SetNextWindowSize(viewport->Size);
+    SetNextWindowPos(viewport->GetWorkPos());
+    SetNextWindowSize(viewport->GetWorkSize());
     SetNextWindowViewport(viewport->ID);
 
     ImGuiWindowFlags host_window_flags = 0;
@@ -15230,7 +15241,10 @@ void ImGui::ShowMetricsWindow(bool* p_open)
             if (ImGui::TreeNode((void*)(intptr_t)viewport->ID, "Viewport #%d, ID: 0x%08X, Parent: 0x%08X, Window: \"%s\"", viewport->Idx, viewport->ID, viewport->ParentViewportId, viewport->Window ? viewport->Window->Name : "N/A"))
             {
                 ImGuiWindowFlags flags = viewport->Flags;
-                ImGui::BulletText("Pos: (%.0f,%.0f), Size: (%.0f,%.0f), Monitor: %d, DpiScale: %.0f%%", viewport->Pos.x, viewport->Pos.y, viewport->Size.x, viewport->Size.y, viewport->PlatformMonitor, viewport->DpiScale * 100.0f);
+                ImGui::BulletText("Main Pos: (%.0f,%.0f), Size: (%.0f,%.0f)\nWorkArea Offset Left: %.0f Top: %.0f, Right: %.0f, Bottom: %.0f\nMonitor: %d, DpiScale: %.0f%%",
+                    viewport->Pos.x, viewport->Pos.y, viewport->Size.x, viewport->Size.y,
+                    viewport->WorkOffsetMin.x, viewport->WorkOffsetMin.y, viewport->WorkOffsetMax.x, viewport->WorkOffsetMax.y,
+                    viewport->PlatformMonitor, viewport->DpiScale * 100.0f);
                 if (viewport->Idx > 0) { ImGui::SameLine(); if (ImGui::SmallButton("Reset Pos")) { viewport->Pos = ImVec2(200,200); if (viewport->Window) viewport->Window->Pos = ImVec2(200,200); } }
                 ImGui::BulletText("Flags: 0x%04X =%s%s%s%s%s%s%s", viewport->Flags,
                     (flags & ImGuiViewportFlags_CanHostOtherWindows) ? " CanHostOtherWindows" : "", (flags & ImGuiViewportFlags_NoDecoration) ? " NoDecoration" : "",

imgui.h

@@ -2468,14 +2468,18 @@ enum ImGuiViewportFlags_
     ImGuiViewportFlags_CanHostOtherWindows      = 1 << 9    // Main viewport: can host multiple imgui windows (secondary viewports are associated to a single window).
 };
 
-// The viewports created and managed by imgui. The role of the platform back-end is to create the platform/OS windows corresponding to each viewport.
+// The viewports created and managed by Dear ImGui. The role of the platform back-end is to create the platform/OS windows corresponding to each viewport.
+// - Main Area = entire viewport.
+// - Work Area = entire viewport minus sections optionally used by menu bars, status bars. Some positioning code will prefer to use this. Window are also trying to stay within this area.
 struct ImGuiViewport
 {
     ImGuiID             ID;                     // Unique identifier for the viewport
     ImGuiViewportFlags  Flags;                  // See ImGuiViewportFlags_
-    ImVec2              Pos;                    // Position of viewport both in imgui space and in OS desktop/native space
-    ImVec2              Size;                   // Size of viewport in pixel
-    float               DpiScale;               // 1.0f = 96 DPI = No extra scale
+    ImVec2              Pos;                    // Main Area: Position of the viewport (the imgui coordinates are the same as OS desktop/native coordinates)
+    ImVec2              Size;                   // Main Area: Size of the viewport.
+    ImVec2              WorkOffsetMin;          // Work Area: Offset from Pos to top-left corner of Work Area. Generally (0,0) or (0,+main_menu_bar_height). Work Area is Full Area but without menu-bars/status-bars (so WorkArea always fit inside Pos/Size!)
+    ImVec2              WorkOffsetMax;          // Work Area: Offset from Pos+Size to bottom-right corner of Work Area. Generally (0,0) or (0,-status_bar_height).
+    float               DpiScale;               // 1.0f = 96 DPI = No extra scale.
     ImDrawData*         DrawData;               // The ImDrawData corresponding to this viewport. Valid after Render() and until the next call to NewFrame().
     ImGuiID             ParentViewportId;       // (Advanced) 0: no parent. Instruct the platform back-end to setup a parent/child relationship between platform windows.
 
@@ -2491,6 +2495,10 @@ struct ImGuiViewport
 
     ImGuiViewport()     { ID = 0; Flags = 0; DpiScale = 0.0f; DrawData = NULL; ParentViewportId = 0; RendererUserData = PlatformUserData = PlatformHandle = PlatformHandleRaw = NULL; PlatformRequestMove = PlatformRequestResize = PlatformRequestClose = false; }
     ~ImGuiViewport()    { IM_ASSERT(PlatformUserData == NULL && RendererUserData == NULL); }
+
+    // Access work-area rectangle
+    ImVec2              GetWorkPos()            { return ImVec2(Pos.x + WorkOffsetMin.x, Pos.y + WorkOffsetMin.y); }
+    ImVec2              GetWorkSize()           { return ImVec2(Size.x - WorkOffsetMin.x + WorkOffsetMax.x, Size.y - WorkOffsetMin.y + WorkOffsetMax.y); } // This not clamped
 };
 
 #if defined(__clang__)

imgui_demo.cpp

@@ -222,9 +222,9 @@ void ImGui::ShowDemoWindow(bool* p_open)
     IM_ASSERT(ImGui::GetCurrentContext() != NULL && "Missing dear imgui context. Refer to examples app!"); // Exceptionally add an extra assert here for people confused with initial dear imgui setup
 
     // Examples Apps (accessible from the "Examples" menu)
+    static bool show_app_main_menu_bar = false;
     static bool show_app_dockspace = false;
     static bool show_app_documents = false;
-    static bool show_app_main_menu_bar = false;
     static bool show_app_console = false;
     static bool show_app_log = false;
     static bool show_app_layout = false;
@@ -236,9 +236,9 @@ void ImGui::ShowDemoWindow(bool* p_open)
     static bool show_app_window_titles = false;
     static bool show_app_custom_rendering = false;
 
+    if (show_app_main_menu_bar)       ShowExampleAppMainMenuBar();
     if (show_app_dockspace)           ShowExampleAppDockSpace(&show_app_dockspace);     // Process the Docking app first, as explicit DockSpace() nodes needs to be submitted early (read comments near the DockSpace function)
     if (show_app_documents)           ShowExampleAppDocuments(&show_app_documents);     // Process the Document app next, as it may also use a DockSpace()
-    if (show_app_main_menu_bar)       ShowExampleAppMainMenuBar();
     if (show_app_console)             ShowExampleAppConsole(&show_app_console);
     if (show_app_log)                 ShowExampleAppLog(&show_app_log);
     if (show_app_layout)              ShowExampleAppLayout(&show_app_layout);
@@ -286,8 +286,8 @@ void ImGui::ShowDemoWindow(bool* p_open)
     if (no_close)           p_open = NULL; // Don't pass our bool* to Begin
 
     // We specify a default position/size in case there's no data in the .ini file. Typically this isn't required! We only do it to make the Demo applications a little more welcoming.
-    ImVec2 main_viewport_pos = ImGui::GetMainViewport()->Pos;
-    ImGui::SetNextWindowPos(ImVec2(main_viewport_pos.x + 650, main_viewport_pos.y + 20), ImGuiCond_FirstUseEver);
+    ImGuiViewport* main_viewport = ImGui::GetMainViewport();
+    ImGui::SetNextWindowPos(ImVec2(main_viewport->GetWorkPos().x + 650, main_viewport->GetWorkPos().y + 20), ImGuiCond_FirstUseEver);
     ImGui::SetNextWindowSize(ImVec2(550, 680), ImGuiCond_FirstUseEver);
 
     // Main body of the Demo window starts here.
@@ -4458,7 +4458,9 @@ static void ShowExampleAppSimpleOverlay(bool* p_open)
     if (corner != -1)
     {
         ImGuiViewport* viewport = ImGui::GetMainViewport();
-        ImVec2 window_pos = ImVec2((corner & 1) ? (viewport->Pos.x + viewport->Size.x - DISTANCE) : (viewport->Pos.x + DISTANCE), (corner & 2) ? (viewport->Pos.y + viewport->Size.y - DISTANCE) : (viewport->Pos.y + DISTANCE));
+        ImVec2 work_area_pos = viewport->GetWorkPos();   // Instead of using viewport->Pos we use GetWorkPos() to avoid menu bars, if any!
+        ImVec2 work_area_size = viewport->GetWorkSize();
+        ImVec2 window_pos = ImVec2((corner & 1) ? (work_area_pos.x + work_area_size.x - DISTANCE) : (work_area_pos.x + DISTANCE), (corner & 2) ? (work_area_pos.y + work_area_size.y - DISTANCE) : (work_area_pos.y + DISTANCE));
         ImVec2 window_pos_pivot = ImVec2((corner & 1) ? 1.0f : 0.0f, (corner & 2) ? 1.0f : 0.0f);
         ImGui::SetNextWindowPos(window_pos, ImGuiCond_Always, window_pos_pivot);
         ImGui::SetNextWindowViewport(viewport->ID);
@@ -4714,8 +4716,8 @@ void ShowExampleAppDockSpace(bool* p_open)
     if (opt_fullscreen)
     {
         ImGuiViewport* viewport = ImGui::GetMainViewport();
-        ImGui::SetNextWindowPos(viewport->Pos);
-        ImGui::SetNextWindowSize(viewport->Size);
+        ImGui::SetNextWindowPos(viewport->GetWorkPos());
+        ImGui::SetNextWindowSize(viewport->GetWorkSize());
         ImGui::SetNextWindowViewport(viewport->ID);
         ImGui::PushStyleVar(ImGuiStyleVar_WindowRounding, 0.0f);
         ImGui::PushStyleVar(ImGuiStyleVar_WindowBorderSize, 0.0f);

imgui_internal.h

@@ -934,10 +934,13 @@ struct ImGuiViewportP : public ImGuiViewport
     ImVec2              LastPlatformPos;
     ImVec2              LastPlatformSize;
     ImVec2              LastRendererSize;
+    ImVec2              CurrWorkOffsetMin;        // Work area top-left offset being increased during the frame
+    ImVec2              CurrWorkOffsetMax;        // Work area bottom-right offset being decreased during the frame
 
     ImGuiViewportP()                { Idx = -1; LastFrameActive = LastFrameDrawLists[0] = LastFrameDrawLists[1] = LastFrontMostStampCount = -1; LastNameHash = 0; Alpha = LastAlpha = 1.0f; PlatformMonitor = -1; PlatformWindowCreated = false; Window = NULL; DrawLists[0] = DrawLists[1] = NULL; LastPlatformPos = LastPlatformSize = LastRendererSize = ImVec2(FLT_MAX, FLT_MAX); }
     ~ImGuiViewportP()               { if (DrawLists[0]) IM_DELETE(DrawLists[0]); if (DrawLists[1]) IM_DELETE(DrawLists[1]); }
     ImRect  GetMainRect() const     { return ImRect(Pos.x, Pos.y, Pos.x + Size.x, Pos.y + Size.y); }
+    ImRect  GetWorkRect() const     { return ImRect(Pos.x + WorkOffsetMin.x, Pos.y + WorkOffsetMin.y, Pos.x + Size.x + WorkOffsetMax.x, Pos.y + Size.y + WorkOffsetMax.y); }
     void    ClearRequestFlags()     { PlatformRequestClose = PlatformRequestMove = PlatformRequestResize = false; }
 };
 
@@ -991,7 +994,7 @@ struct ImGuiNextWindowData
     ImGuiID                     ViewportId;
     ImGuiID                     DockId;
     ImGuiWindowClass            WindowClass;
-    ImVec2                      MenuBarOffsetMinVal;    // *Always on* This is not exposed publicly, so we don't clear it.
+    ImVec2                      MenuBarOffsetMinVal;    // (Always on) This is not exposed publicly, so we don't clear it and it doesn't have a corresponding flag (could we? for consistency?)
 
     ImGuiNextWindowData()       { memset(this, 0, sizeof(*this)); }
     inline void ClearFlags()    { Flags = ImGuiNextWindowDataFlags_None; }

imgui_widgets.cpp

@@ -6182,20 +6182,32 @@ void ImGui::EndMenuBar()
     window->DC.MenuBarAppending = false;
 }
 
-// For the main menu bar, which cannot be moved, we honor g.Style.DisplaySafeAreaPadding to ensure text can be visible on a TV set.
 bool ImGui::BeginMainMenuBar()
 {
     ImGuiContext& g = *GImGui;
-    ImGuiViewport* viewport = g.Viewports[0];
+    ImGuiViewportP* viewport = g.Viewports[0];
+
+    // For the main menu bar, which cannot be moved, we honor g.Style.DisplaySafeAreaPadding to ensure text can be visible on a TV set.
     g.NextWindowData.MenuBarOffsetMinVal = ImVec2(g.Style.DisplaySafeAreaPadding.x, ImMax(g.Style.DisplaySafeAreaPadding.y - g.Style.FramePadding.y, 0.0f));
-    SetNextWindowPos(viewport->Pos);
-    SetNextWindowSize(ImVec2(viewport->Size.x, g.NextWindowData.MenuBarOffsetMinVal.y + g.FontBaseSize + g.Style.FramePadding.y));
-    SetNextWindowViewport(viewport->ID); // Enforce viewport so we don't create our onw viewport when ImGuiConfigFlags_ViewportsNoMerge is set.
+
+    // Get our rectangle in the work area, and report the size we need for next frame.
+    // We don't attempt to calculate our height ahead, as it depends on the per-viewport font size. However menu-bar will affect the minimum window size so we'll get the right height.
+    ImVec2 menu_bar_pos = viewport->Pos + viewport->CurrWorkOffsetMin;
+    ImVec2 menu_bar_size = ImVec2(viewport->Size.x - viewport->CurrWorkOffsetMin.x + viewport->CurrWorkOffsetMax.x, 1.0f);
+
+    // Create window
+    SetNextWindowPos(menu_bar_pos);
+    SetNextWindowSize(menu_bar_size);
+    SetNextWindowViewport(viewport->ID); // Enforce viewport so we don't create our own viewport when ImGuiConfigFlags_ViewportsNoMerge is set.
     PushStyleVar(ImGuiStyleVar_WindowRounding, 0.0f);
-    PushStyleVar(ImGuiStyleVar_WindowMinSize, ImVec2(0, 0));
+    PushStyleVar(ImGuiStyleVar_WindowMinSize, ImVec2(0, 0));    // Lift normal size constraint, however the presence of a menu-bar will give us the minimum height we want.
     ImGuiWindowFlags window_flags = ImGuiWindowFlags_NoDocking | ImGuiWindowFlags_NoTitleBar | ImGuiWindowFlags_NoResize | ImGuiWindowFlags_NoMove | ImGuiWindowFlags_NoScrollbar | ImGuiWindowFlags_NoSavedSettings | ImGuiWindowFlags_MenuBar;
     bool is_open = Begin("##MainMenuBar", NULL, window_flags) && BeginMenuBar();
     PopStyleVar(2);
+
+    // Feed back into work area using actual window size
+    viewport->CurrWorkOffsetMin.y += GetCurrentWindow()->Size.y;
+
     g.NextWindowData.MenuBarOffsetMinVal = ImVec2(0.0f, 0.0f);
     if (!is_open)
     {