Window: Added ImGuiWindowFlags_NoBackground flag for consistency and to ease creating new flag combinations. Added ImGuiWindowFlags_NoDecoration helper flag which is essentially NoTitleBar+NoResize+NoScrollbar+NoCollapse. (#1660)

docs/CHANGELOG.txt

@@ -53,6 +53,9 @@ Other Changes:
 - Window: Resizing from edges (with io.ConfigResizeWindowsFromEdges Beta flag) extends the hit region
   of root floating windows outside the window, making it easier to resize windows. Resize grips are also
   extended accordingly so there are no discontinuity when hovering between borders and corners. (#1495, #822)
+- Window: Added ImGuiWindowFlags_NoBackground flag to avoid rendering window background. This is mostly to allow
+  the creation of new flag combinations, as we could already use SetNextWindowBgAlpha(0.0f). (#1660) [@biojppm, @ocornut]
+- Window: Added ImGuiWindowFlags_NoDecoration helper flag which is essentially NoTitleBar+NoResize+NoScrollbar+NoCollapse.
 - Window, Inputs: Fixed resizing from edges when io.MousePos is not pixel-rounded by rounding mouse position input. (#2110)
 - BeginChild(): Fixed BeginChild(const char*, ...) variation erroneously not applying the ID stack
   to the provided string to uniquely identify the child window. This was undoing an intentional change

imgui.cpp

@@ -4901,13 +4901,14 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         else
         {
             // Window background
+            if (!(flags & ImGuiWindowFlags_NoBackground))
+            {
                 ImU32 bg_col = GetColorU32(GetWindowBgColorIdxFromFlags(flags));
                 if (g.NextWindowData.BgAlphaCond != 0)
-            {
                     bg_col = (bg_col & ~IM_COL32_A_MASK) | (IM_F32_TO_INT8_SAT(g.NextWindowData.BgAlphaVal) << IM_COL32_A_SHIFT);
-                g.NextWindowData.BgAlphaCond = 0;
-            }
                 window->DrawList->AddRectFilled(window->Pos + ImVec2(0, window->TitleBarHeight()), window->Pos + window->Size, bg_col, window_rounding, (flags & ImGuiWindowFlags_NoTitleBar) ? ImDrawCornerFlags_All : ImDrawCornerFlags_Bot);
+            }
+            g.NextWindowData.BgAlphaCond = 0;
 
             // Title bar
             ImU32 title_bar_col = GetColorU32(window->Collapsed ? ImGuiCol_TitleBgCollapsed : title_bar_is_highlight ? ImGuiCol_TitleBgActive : ImGuiCol_TitleBg);
@@ -4945,7 +4946,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
             }
 
             // Borders
-            if (window_border_size > 0.0f)
+            if (window_border_size > 0.0f && !(flags & ImGuiWindowFlags_NoBackground))
                 window->DrawList->AddRect(window->Pos, window->Pos + window->Size, GetColorU32(ImGuiCol_Border), window_rounding, ImDrawCornerFlags_All, window_border_size);
             if (border_held != -1)
             {

imgui.h

@@ -229,7 +229,7 @@ namespace ImGui
     IMGUI_API void          SetNextWindowContentSize(const ImVec2& size);                               // set next window content size (~ enforce the range of scrollbars). not including window decorations (title bar, menu bar, etc.). set an axis to 0.0f to leave it automatic. call before Begin()
     IMGUI_API void          SetNextWindowCollapsed(bool collapsed, ImGuiCond cond = 0);                 // set next window collapsed state. call before Begin()
     IMGUI_API void          SetNextWindowFocus();                                                       // set next window to be focused / front-most. call before Begin()
-    IMGUI_API void          SetNextWindowBgAlpha(float alpha);                                          // set next window background color alpha. helper to easily modify ImGuiCol_WindowBg/ChildBg/PopupBg.
+    IMGUI_API void          SetNextWindowBgAlpha(float alpha);                                          // set next window background color alpha. helper to easily modify ImGuiCol_WindowBg/ChildBg/PopupBg. you may also use ImGuiWindowFlags_NoBackground.
     IMGUI_API void          SetWindowPos(const ImVec2& pos, ImGuiCond cond = 0);                        // (not recommended) set current window position - call within Begin()/End(). prefer using SetNextWindowPos(), as this may incur tearing and side-effects.
     IMGUI_API void          SetWindowSize(const ImVec2& size, ImGuiCond cond = 0);                      // (not recommended) set current window size - call within Begin()/End(). set to ImVec2(0,0) to force an auto-fit. prefer using SetNextWindowSize(), as this may incur tearing and minor side-effects.
     IMGUI_API void          SetWindowCollapsed(bool collapsed, ImGuiCond cond = 0);                     // (not recommended) set current window collapsed state. prefer using SetNextWindowCollapsed().
@@ -620,6 +620,7 @@ enum ImGuiWindowFlags_
     ImGuiWindowFlags_NoScrollWithMouse      = 1 << 4,   // Disable user vertically scrolling with mouse wheel. On child window, mouse wheel will be forwarded to the parent unless NoScrollbar is also set.
     ImGuiWindowFlags_NoCollapse             = 1 << 5,   // Disable user collapsing window by double-clicking on it
     ImGuiWindowFlags_AlwaysAutoResize       = 1 << 6,   // Resize every window to its content every frame
+    ImGuiWindowFlags_NoBackground           = 1 << 7,   // Disable drawing background color (WindowBg, etc.) and outside border. Similar as using SetNextWindowBgAlpha(0.0f).
     ImGuiWindowFlags_NoSavedSettings        = 1 << 8,   // Never load/save settings in .ini file
     ImGuiWindowFlags_NoInputs               = 1 << 9,   // Disable catching mouse or keyboard inputs, hovering test with pass through.
     ImGuiWindowFlags_MenuBar                = 1 << 10,  // Has a menu-bar
@@ -632,6 +633,7 @@ enum ImGuiWindowFlags_
     ImGuiWindowFlags_NoNavInputs            = 1 << 18,  // No gamepad/keyboard navigation within the window
     ImGuiWindowFlags_NoNavFocus             = 1 << 19,  // No focusing toward this window with gamepad/keyboard navigation (e.g. skipped by CTRL+TAB)
     ImGuiWindowFlags_NoNav                  = ImGuiWindowFlags_NoNavInputs | ImGuiWindowFlags_NoNavFocus,
+    ImGuiWindowFlags_NoDecoration           = ImGuiWindowFlags_NoTitleBar | ImGuiWindowFlags_NoResize | ImGuiWindowFlags_NoScrollbar | ImGuiWindowFlags_NoCollapse,
 
     // [Internal]
     ImGuiWindowFlags_NavFlattened           = 1 << 23,  // [BETA] Allow gamepad/keyboard navigation to cross over parent border to this child (only use on child that have no scrolling!)

imgui_demo.cpp

@@ -207,6 +207,7 @@ void ImGui::ShowDemoWindow(bool* p_open)
     static bool no_collapse = false;
     static bool no_close = false;
     static bool no_nav = false;
+    static bool no_background = false;
     static bool no_bring_to_front = false;
     
     ImGuiWindowFlags window_flags = 0;
@@ -217,6 +218,7 @@ void ImGui::ShowDemoWindow(bool* p_open)
     if (no_resize)          window_flags |= ImGuiWindowFlags_NoResize;
     if (no_collapse)        window_flags |= ImGuiWindowFlags_NoCollapse;
     if (no_nav)             window_flags |= ImGuiWindowFlags_NoNav;
+    if (no_background)      window_flags |= ImGuiWindowFlags_NoBackground;
     if (no_bring_to_front)  window_flags |= ImGuiWindowFlags_NoBringToFrontOnFocus;
     if (no_close)           p_open = NULL; // Don't pass our bool* to Begin
 
@@ -362,6 +364,7 @@ void ImGui::ShowDemoWindow(bool* p_open)
         ImGui::Checkbox("No collapse", &no_collapse);
         ImGui::Checkbox("No close", &no_close); ImGui::SameLine(150);
         ImGui::Checkbox("No nav", &no_nav); ImGui::SameLine(300);
+        ImGui::Checkbox("No background", &no_background);
         ImGui::Checkbox("No bring to front", &no_bring_to_front);
     }