CollapseButton: handle clipping better + align circle like in docking branch.

Amend 6c3697f6

imgui_widgets.cpp

@@ -809,7 +809,7 @@ bool ImGui::CloseButton(ImGuiID id, const ImVec2& pos)
         bb_interact.Expand(ImFloor(bb_interact.GetSize() * -0.25f));
 
     // Tweak 2: We intentionally allow interaction when clipped so that a mechanical Alt,Right,Activate sequence can always close a window.
-    // (this isn't the regular behavior of buttons, but it doesn't affect the user much because navigation tends to keep items visible).
+    // (this isn't the common behavior of buttons, but it doesn't affect the user because navigation tends to keep items visible in scrolling layer).
     bool is_clipped = !ItemAdd(bb_interact, id);
 
     bool hovered, held;
@@ -839,15 +839,17 @@ bool ImGui::CollapseButton(ImGuiID id, const ImVec2& pos)
     ImGuiWindow* window = g.CurrentWindow;
 
     ImRect bb(pos, pos + ImVec2(g.FontSize, g.FontSize) + g.Style.FramePadding * 2.0f);
-    ItemAdd(bb, id);
+    bool is_clipped = !ItemAdd(bb, id);
     bool hovered, held;
     bool pressed = ButtonBehavior(bb, id, &hovered, &held, ImGuiButtonFlags_None);
+    if (is_clipped)
+        return pressed;
 
     // Render
     ImU32 bg_col = GetColorU32((held && hovered) ? ImGuiCol_ButtonActive : hovered ? ImGuiCol_ButtonHovered : ImGuiCol_Button);
     ImU32 text_col = GetColorU32(ImGuiCol_Text);
     if (hovered || held)
-        window->DrawList->AddCircleFilled(bb.GetCenter()/*+ ImVec2(0.0f, -0.5f)*/, g.FontSize * 0.5f + 1.0f, bg_col);
+        window->DrawList->AddCircleFilled(bb.GetCenter() + ImVec2(0.0f, -0.5f), g.FontSize * 0.5f + 1.0f, bg_col);
     RenderArrow(window->DrawList, bb.Min + g.Style.FramePadding, text_col, window->Collapsed ? ImGuiDir_Right : ImGuiDir_Down, 1.0f);
 
     // Switch to moving the window after mouse is moved beyond the initial drag threshold