This question is both philosophical and practical. When you say "I need to know constantly...", you might want to evaluate what you really mean by "constantly" (on a nanosecondly basis? more often than that? is every 1/44,100 of a second enough, or every millisecond? or every 50 ms? or "whenever x happens"? if so, how soon after x happens do you need to know?) and what you really mean by "need" (if you did have "constant" knowledge, how often would you actually access that information?). The answer to those sorts of questions will tell you when, and by what means, you should trigger the 'getpos' and 'getsize' messages.

Thanks for your answer.

Well, you're right, and I definitely can send "getpos" and "getsize" from time to time. It's what I do at the moment, and it works fine. I just thought there might be a mechanism to be warned every time the window is resized/moved. Maybe the [route position positionidle] of the help patch is supposed to give that information (?), but I can't get it to work.