Scrollbar: rounding corners selected based on a generic helper.

CalcRoundingFlagsForRectInRect() is backported and used multiple times by docking.

imgui_draw.cpp

diff --git a/imgui_draw.cpp b/imgui_draw.cpp
index 00e0583..5c07676 100644
--- a/imgui_draw.cpp
+++ b/imgui_draw.cpp
@@ -6114,6 +6114,17 @@
     if (fill_R && fill_D) draw_list->AddRectFilled(ImVec2(inner.Max.x, inner.Max.y), ImVec2(outer.Max.x, outer.Max.y), col, rounding, ImDrawFlags_RoundCornersBottomRight);
 }
 
+ImDrawFlags ImGui::CalcRoundingFlagsForRectInRect(const ImRect& r_in, const ImRect& r_outer, float threshold)
+{
+    bool round_l = r_in.Min.x <= r_outer.Min.x + threshold;
+    bool round_r = r_in.Max.x >= r_outer.Max.x - threshold;
+    bool round_t = r_in.Min.y <= r_outer.Min.y + threshold;
+    bool round_b = r_in.Max.y >= r_outer.Max.y - threshold;
+    return ImDrawFlags_RoundCornersNone
+        | ((round_t && round_l) ? ImDrawFlags_RoundCornersTopLeft : 0) | ((round_t && round_r) ? ImDrawFlags_RoundCornersTopRight : 0)
+        | ((round_b && round_l) ? ImDrawFlags_RoundCornersBottomLeft : 0) | ((round_b && round_r) ? ImDrawFlags_RoundCornersBottomRight : 0);
+}
+
 // Helper for ColorPicker4()
 // NB: This is rather brittle and will show artifact when rounding this enabled if rounded corners overlap multiple cells. Caller currently responsible for avoiding that.
 // Spent a non reasonable amount of time trying to getting this right for ColorButton with rounding+anti-aliasing+ImGuiColorEditFlags_HalfAlphaPreview flag + various grid sizes and offsets, and eventually gave up... probably more reasonable to disable rounding altogether.

imgui_internal.h

diff --git a/imgui_internal.h b/imgui_internal.h
index c17873c..b0da43d 100644
--- a/imgui_internal.h
+++ b/imgui_internal.h
@@ -3606,6 +3606,7 @@
     IMGUI_API void          RenderArrowPointingAt(ImDrawList* draw_list, ImVec2 pos, ImVec2 half_sz, ImGuiDir direction, ImU32 col);
     IMGUI_API void          RenderRectFilledInRangeH(ImDrawList* draw_list, const ImRect& rect, ImU32 col, float fill_x0, float fill_x1, float rounding);
     IMGUI_API void          RenderRectFilledWithHole(ImDrawList* draw_list, const ImRect& outer, const ImRect& inner, ImU32 col, float rounding);
+    IMGUI_API ImDrawFlags   CalcRoundingFlagsForRectInRect(const ImRect& r_in, const ImRect& r_outer, float threshold);
 
     // Widgets: Text
     IMGUI_API void          TextEx(const char* text, const char* text_end = NULL, ImGuiTextFlags flags = 0);

imgui_widgets.cpp

diff --git a/imgui_widgets.cpp b/imgui_widgets.cpp
index 8911863..0fabf79 100644
--- a/imgui_widgets.cpp
+++ b/imgui_widgets.cpp
@@ -982,20 +982,7 @@
 
     // Calculate scrollbar bounding box
     ImRect bb = GetWindowScrollbarRect(window, axis);
-    ImDrawFlags rounding_corners = ImDrawFlags_RoundCornersNone;
-    if (axis == ImGuiAxis_X)
-    {
-        rounding_corners |= ImDrawFlags_RoundCornersBottomLeft;
-        if (!window->ScrollbarY)
-            rounding_corners |= ImDrawFlags_RoundCornersBottomRight;
-    }
-    else
-    {
-        if ((window->Flags & ImGuiWindowFlags_NoTitleBar) && !(window->Flags & ImGuiWindowFlags_MenuBar))
-            rounding_corners |= ImDrawFlags_RoundCornersTopRight;
-        if (!window->ScrollbarX)
-            rounding_corners |= ImDrawFlags_RoundCornersBottomRight;
-    }
+    ImDrawFlags rounding_corners = CalcRoundingFlagsForRectInRect(bb, window->Rect(), g.Style.WindowBorderSize);
     float size_visible = window->InnerRect.Max[axis] - window->InnerRect.Min[axis];
     float size_contents = window->ContentSize[axis] + window->WindowPadding[axis] * 2.0f;
     ImS64 scroll = (ImS64)window->Scroll[axis];