Window: Fix SetNextWindowBgAlpha(1.0f) failing to override alpha component. (#3007)

docs/CHANGELOG.txt

@@ -72,6 +72,7 @@ Other Changes:
 - ColorEdit: "Copy As" context-menu tool shows hex values with a '#' prefix instead of '0x'.
 - ColorEdit: "Copy As" content-menu tool shows hex values both with/without alpha when available.
 - MenuBar: Fix minor clipping issue where occasionally a menu text can overlap the right-most border.
+- Window: Fix SetNextWindowBgAlpha(1.0f) failing to override alpha component. (#3007) [@Albog]
 - ImDrawList: AddCircle(), AddCircleFilled() API can now auto-tessellate when provided a segment
   count of zero. Alter tessellation quality with 'style.CircleSegmentMaxError'. [@ShironekoBen]
 - ImDrawList: Add AddNgon(), AddNgonFilled() API with a guarantee on the explicit segment count.

imgui.cpp

@@ -5161,10 +5161,14 @@ void ImGui::RenderWindowDecorations(ImGuiWindow* window, const ImRect& title_bar
         if (!(flags & ImGuiWindowFlags_NoBackground))
         {
             ImU32 bg_col = GetColorU32(GetWindowBgColorIdxFromFlags(flags));
+            bool override_alpha = false;
             float alpha = 1.0f;
             if (g.NextWindowData.Flags & ImGuiNextWindowDataFlags_HasBgAlpha)
+            {
                 alpha = g.NextWindowData.BgAlphaVal;
-            if (alpha != 1.0f)
+                override_alpha = true;
+            }
+            if (override_alpha)
                 bg_col = (bg_col & ~IM_COL32_A_MASK) | (IM_F32_TO_INT8_SAT(alpha) << IM_COL32_A_SHIFT);
             window->DrawList->AddRectFilled(window->Pos + ImVec2(0, window->TitleBarHeight()), window->Pos + window->Size, bg_col, window_rounding, (flags & ImGuiWindowFlags_NoTitleBar) ? ImDrawCornerFlags_All : ImDrawCornerFlags_Bot);
         }

imgui.h

@@ -294,7 +294,7 @@ namespace ImGui
     IMGUI_API void          SetNextWindowContentSize(const ImVec2& size);                               // set next window content size (~ scrollable client area, which enforce the range of scrollbars). Not including window decorations (title bar, menu bar, etc.) nor WindowPadding. 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 / top-most. call before Begin()
-    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          SetNextWindowBgAlpha(float alpha);                                          // set next window background color alpha. helper to easily override the Alpha component of 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().

imgui_internal.h

@@ -939,7 +939,7 @@ struct ImGuiNextWindowData
     ImRect                      SizeConstraintRect;
     ImGuiSizeCallback           SizeCallback;
     void*                       SizeCallbackUserData;
-    float                       BgAlphaVal;
+    float                       BgAlphaVal;             // Override background alpha
     ImVec2                      MenuBarOffsetMinVal;    // *Always on* This is not exposed publicly, so we don't clear it.
 
     ImGuiNextWindowData()       { memset(this, 0, sizeof(*this)); }