DoHookClipRects(): How to draw LAYERSMART obsured parts with correct offset ?

I have a very perverse glitch when using a DoHookClipRects() draw hook, to render my glyphs with utf8rastport, my lib to render texts and emojis.
The bug happen OS3.2.3 UAE P96.)
I have to use that function in P96/CGX RTG modes where I need to blend directly RGBA pixels with the CPU on the screen bitmap.

The hook is meant to have this struct passed to a1:
struct bf_msg {
struct Layer *bf_Layer;
struct Rectangle bf_Bounds;
LONG bf_OffsetX;
LONG bf_OffsetY;
};

In the hook, I apply the rastport top-left origin offset to the drawing. I get it with msg->bf_Layer->bounds.MinX /MinY
...then I add that to the bitmap, clip to msg->bf_bounds, and draw:

It works perfectly for the "rectangle parts" that are visible, but I have a boopsi LAYERSMART window, and it looks like
it also calls the same hook for the "backup rectangle" that are obscured by other windows in front, and in that case the offset given with msg->bf_Layer->bounds.MinX/Y is wrong, and of course because it's an hidden part of the rectangle, so it shouldn't even been written on the screen bitmap, but well when you move the window that obscure the part, the part that was hidden is refreshed with "an offset error", like if msg->bf_Layer->bounds was applied 2 times...

So it looks like In the hook draw function I have to test if I have to draw on an "obscured" window part , and not apply the offset, or the same offset, in that case... brrrr something like that, no idea.
code in urp_cgx_clip_hook_func(), https://github.com/krabobmkd/EmojiGear/blob/main/libutf8rastport/utf8ra… , l. 1742.

By the way, I can't find a single example of a hook function to be used with DoHookClipRects() on the whole planet