Docking: docked windows honor ImGuiCol_WindowBg. Host window in charge of rendering seams. (#2700, #2539 + Docked windows honor display their border properly. (#2522)

Plus: better support for transparent one in nodes
Side effects: DockContextBindNodeToWindow doesn't alter node->IsVisible.
Side effects: ImDrawList:: _ResetForNewFrame() needs to merge, sane (in case of
(Amended, force-pushed)

docs/CHANGELOG.txt

@@ -158,6 +158,8 @@ Docking+Viewports Branch:
 - Docking: Revert removal of io.ConfigDockingWithShift config option (removed in 1.83). (#4643)
 - Docking: Fixed a bug undocking windows docked into a non-visible or _KeepAliveOnly dockspace
   when unrelated windows submitted before the dockspace have dynamic visibility. (#4757)
+- Docking, Style: Docked windows honor ImGuiCol_WindowBg. (#2700, #2539)
+- Docking, Style: Docked windows honor display their border properly. (#2522)
 - Docking: Fixed incorrectly rounded tab bars for dock node that are not at the top of their dock tree.
 - Docking: Fixed single-frame node pos/size inconsistencies when window stop or start being submitted.
 - Viewports: Made it possible to explicitly assign ImGuiWindowClass::ParentViewportId to 0 in order

imgui_draw.cpp

@@ -412,6 +412,8 @@ void ImDrawList::_ResetForNewFrame()
     IM_ASSERT(IM_OFFSETOF(ImDrawCmd, ClipRect) == 0);
     IM_ASSERT(IM_OFFSETOF(ImDrawCmd, TextureId) == sizeof(ImVec4));
     IM_ASSERT(IM_OFFSETOF(ImDrawCmd, VtxOffset) == sizeof(ImVec4) + sizeof(ImTextureID));
+    if (_Splitter._Count > 1)
+        _Splitter.Merge(this);
 
     CmdBuffer.resize(0);
     IdxBuffer.resize(0);

imgui_internal.h

@@ -1432,6 +1432,7 @@ struct IMGUI_API ImGuiDockNode
     ImVec2                  SizeRef;                    // [Split node only] Last explicitly written-to size (overridden when using a splitter affecting the node), used to calculate Size.
     ImGuiAxis               SplitAxis;                  // [Split node only] Split axis (X or Y)
     ImGuiWindowClass        WindowClass;                // [Root node only]
+    ImU32                   LastBgColor;
 
     ImGuiWindow*            HostWindow;
     ImGuiWindow*            VisibleWindow;              // Generally point to window which is ID is == SelectedTabID, but when CTRL+Tabbing this can be a different window.
@@ -1449,6 +1450,7 @@ struct IMGUI_API ImGuiDockNode
     ImGuiDataAuthority      AuthorityForViewport    :3;
     bool                    IsVisible               :1; // Set to false when the node is hidden (usually disabled as it has no active window)
     bool                    IsFocused               :1;
+    bool                    IsBgDrawnThisFrame      :1;
     bool                    HasCloseButton          :1; // Provide space for a close button (if any of the docked window has one). Note that button may be hidden on window without one.
     bool                    HasWindowMenuButton     :1;
     bool                    HasCentralNodeChild     :1;
@@ -2828,6 +2830,7 @@ namespace ImGui
     IMGUI_API void          DockContextRebuildNodes(ImGuiContext* ctx);
     IMGUI_API void          DockContextNewFrameUpdateUndocking(ImGuiContext* ctx);
     IMGUI_API void          DockContextNewFrameUpdateDocking(ImGuiContext* ctx);
+    IMGUI_API void          DockContextEndFrame(ImGuiContext* ctx);
     IMGUI_API ImGuiID       DockContextGenNodeID(ImGuiContext* ctx);
     IMGUI_API void          DockContextQueueDock(ImGuiContext* ctx, ImGuiWindow* target, ImGuiDockNode* target_node, ImGuiWindow* payload, ImGuiDir split_dir, float split_ratio, bool split_outer);
     IMGUI_API void          DockContextQueueUndockWindow(ImGuiContext* ctx, ImGuiWindow* window);
@@ -3005,7 +3008,7 @@ namespace ImGui
     IMGUI_API bool          ButtonBehavior(const ImRect& bb, ImGuiID id, bool* out_hovered, bool* out_held, ImGuiButtonFlags flags = 0);
     IMGUI_API bool          DragBehavior(ImGuiID id, ImGuiDataType data_type, void* p_v, float v_speed, const void* p_min, const void* p_max, const char* format, ImGuiSliderFlags flags);
     IMGUI_API bool          SliderBehavior(const ImRect& bb, ImGuiID id, ImGuiDataType data_type, void* p_v, const void* p_min, const void* p_max, const char* format, ImGuiSliderFlags flags, ImRect* out_grab_bb);
-    IMGUI_API bool          SplitterBehavior(const ImRect& bb, ImGuiID id, ImGuiAxis axis, float* size1, float* size2, float min_size1, float min_size2, float hover_extend = 0.0f, float hover_visibility_delay = 0.0f);
+    IMGUI_API bool          SplitterBehavior(const ImRect& bb, ImGuiID id, ImGuiAxis axis, float* size1, float* size2, float min_size1, float min_size2, float hover_extend = 0.0f, float hover_visibility_delay = 0.0f, ImU32 bg_col = 0);
     IMGUI_API bool          TreeNodeBehavior(ImGuiID id, ImGuiTreeNodeFlags flags, const char* label, const char* label_end = NULL);
     IMGUI_API bool          TreeNodeBehaviorIsOpen(ImGuiID id, ImGuiTreeNodeFlags flags = 0);                     // Consume previous SetNextItemOpen() data, if any. May return true when logging
     IMGUI_API void          TreePushOverrideID(ImGuiID id);

imgui_widgets.cpp

@@ -1435,7 +1435,7 @@ void ImGui::Separator()
 }
 
 // Using 'hover_visibility_delay' allows us to hide the highlight and mouse cursor for a short time, which can be convenient to reduce visual noise.
-bool ImGui::SplitterBehavior(const ImRect& bb, ImGuiID id, ImGuiAxis axis, float* size1, float* size2, float min_size1, float min_size2, float hover_extend, float hover_visibility_delay)
+bool ImGui::SplitterBehavior(const ImRect& bb, ImGuiID id, ImGuiAxis axis, float* size1, float* size2, float min_size1, float min_size2, float hover_extend, float hover_visibility_delay, ImU32 bg_col)
 {
     ImGuiContext& g = *GImGui;
     ImGuiWindow* window = g.CurrentWindow;
@@ -1487,7 +1487,9 @@ bool ImGui::SplitterBehavior(const ImRect& bb, ImGuiID id, ImGuiAxis axis, float
         }
     }
 
-    // Render
+    // Render at new position
+    if (bg_col & IM_COL32_A_MASK)
+        window->DrawList->AddRectFilled(bb_render.Min, bb_render.Max, bg_col, 0.0f);
     const ImU32 col = GetColorU32(held ? ImGuiCol_SeparatorActive : (hovered && g.HoveredIdTimer >= hover_visibility_delay) ? ImGuiCol_SeparatorHovered : ImGuiCol_Separator);
     window->DrawList->AddRectFilled(bb_render.Min, bb_render.Max, col, 0.0f);