Comment 1 for bug 602653

ToolManager.cs,
this code (on HandlePbToolItemClicked method) :
// Don't let the user unselect the current tool
if (t.Name == CurrentTool.Name) {
 tb.Active = true;
 return;
}
doesn't work as intended, because the odd behaviour when we set ToolItem.Active property.

on SetCurrentTool method :
// Load new tool
index = i;
tool.ToolItem.Active = true; // this code will also invoke Clicked event handler
tool.ToolItem.Active = true; // duplicate code to show the odd. this code doesn't invoke Clicked event handler
tool.DoActivated ();
tool.DoBuildToolBar (PintaCore.Chrome.ToolToolBar);

Maybe it's GTK# bug ?