ImGuiListClipper: Fix to behave within column (#661, #662, #716)

imgui.cpp

@@ -1622,12 +1622,14 @@ float ImGuiSimpleColumns::CalcExtraSpace(float avail_w)
 
 static void SetCursorPosYAndSetupDummyPrevLine(float pos_y, float line_height)
 {
-    // Setting those fields so that SetScrollHere() can properly function after the end of our clipper usage.
-    // If we end up needing more accurate data (to e.g. use SameLine) we may as well make the clipper have a fourth step to let user process and display the last item in their list.
+    // Set cursor position and a few other things so that SetScrollHere() and Columns() can work when seeking cursor. 
+    // FIXME: It is problematic that we have to do that here, because custom/equivalent end-user code would stumble on the same issue. Consider moving within SetCursorXXX functions?
     ImGui::SetCursorPosY(pos_y);
     ImGuiWindow* window = ImGui::GetCurrentWindow();
-    window->DC.CursorPosPrevLine.y = window->DC.CursorPos.y - line_height;
-    window->DC.PrevLineHeight = (line_height - GImGui->Style.ItemSpacing.y);
+    window->DC.CursorPosPrevLine.y = window->DC.CursorPos.y - line_height;      // Setting those fields so that SetScrollHere() can properly function after the end of our clipper usage.
+    window->DC.PrevLineHeight = (line_height - GImGui->Style.ItemSpacing.y);    // If we end up needing more accurate data (to e.g. use SameLine) we may as well make the clipper have a fourth step to let user process and display the last item in their list.
+    if (window->DC.ColumnsCount > 1)
+        window->DC.ColumnsCellMinY = window->DC.CursorPos.y;                    // Setting this so that cell Y position are set properly
 }
 
 // Use case A: Begin() called from constructor with items_height<0, then called again from Sync() in StepNo 1