SetNextWindowContentSize() adjust for client->window size, but the fate of borders isn't really clear for now (until now we always tried to make borders not affect layout, so if we want a 200x200 fully visible space with borders and zero window padding user need to include the borders) (#1490)

imgui.cpp

@@ -4388,7 +4388,9 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
     }
     if (g.SetNextWindowContentSizeCond)
     {
+        // Adjust passed "client size" to become a "window size"
         window->SizeContentsExplicit = g.SetNextWindowContentSizeVal;
+        window->SizeContentsExplicit.y += window->TitleBarHeight() + window->MenuBarHeight();
         g.SetNextWindowContentSizeCond = 0;
     }
     else if (first_begin_of_the_frame)
@@ -5660,7 +5662,7 @@ void ImGui::SetNextWindowSizeConstraints(const ImVec2& size_min, const ImVec2& s
 void ImGui::SetNextWindowContentSize(const ImVec2& size)
 {
     ImGuiContext& g = *GImGui;
-    g.SetNextWindowContentSizeVal = size;
+    g.SetNextWindowContentSizeVal = size;  // In Begin() we will add the size of window decorations (title bar, menu etc.) to that to form a SizeContents value.
     g.SetNextWindowContentSizeCond = ImGuiCond_Always;
 }
 

imgui.h

@@ -159,7 +159,7 @@ namespace ImGui
     IMGUI_API void          SetNextWindowPos(const ImVec2& pos, ImGuiCond cond = 0, const ImVec2& pivot = ImVec2(0,0)); // set next window position. call before Begin(). use pivot=(0.5f,0.5f) to center on given point, etc.
     IMGUI_API void          SetNextWindowSize(const ImVec2& size, ImGuiCond cond = 0);          // set next window size. set axis to 0.0f to force an auto-fit on this axis. call before Begin()
     IMGUI_API void          SetNextWindowSizeConstraints(const ImVec2& size_min, const ImVec2& size_max, ImGuiSizeConstraintCallback custom_callback = NULL, void* custom_callback_data = NULL); // set next window size limits. use -1,-1 on either X/Y axis to preserve the current size. Use callback to apply non-trivial programmatic constraints.
-    IMGUI_API void          SetNextWindowContentSize(const ImVec2& size);                       // set next window content size (~ enforce the range of scrollbars). set axis to 0.0f to leave it automatic. call before Begin()
+    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          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_internal.h

@@ -630,7 +630,7 @@ struct IMGUI_API ImGuiDrawContext
     ImVec2                  CursorPos;
     ImVec2                  CursorPosPrevLine;
     ImVec2                  CursorStartPos;
-    ImVec2                  CursorMaxPos;           // Implicitly calculate the size of our contents, always extending. Saved into window->SizeContents at the end of the frame
+    ImVec2                  CursorMaxPos;           // Used to implicitly calculate the size of our contents, always growing during the frame. Turned into window->SizeContents at the beginning of next frame
     float                   CurrentLineHeight;
     float                   CurrentLineTextBaseOffset;
     float                   PrevLineHeight;
@@ -716,7 +716,7 @@ struct IMGUI_API ImGuiWindow
     ImVec2                  Size;                               // Current size (==SizeFull or collapsed title bar size)
     ImVec2                  SizeFull;                           // Size when non collapsed
     ImVec2                  SizeFullAtLastBegin;                // Copy of SizeFull at the end of Begin. This is the reference value we'll use on the next frame to decide if we need scrollbars.
-    ImVec2                  SizeContents;                       // Size of contents (== extents reach of the drawing cursor) from previous frame
+    ImVec2                  SizeContents;                       // Size of contents (== extents reach of the drawing cursor) from previous frame. Include decoration, window title, border, menu, etc.
     ImVec2                  SizeContentsExplicit;               // Size of contents explicitly set by the user via SetNextWindowContentSize()
     ImRect                  ContentsRegionRect;                 // Maximum visible content position in window coordinates. ~~ (SizeContentsExplicit ? SizeContentsExplicit : Size - ScrollbarSizes) - CursorStartPos, per axis
     ImVec2                  WindowPadding;                      // Window padding at the time of begin.