Curious issue with rectangles on a surface

I drew a simple rectangular block object. On the top-surface of the block I drew a rectangle indexed off the top-left corner. I drew a second rectangle indexed off the lower-right corner of the first rectangle, only in this case the second rectangle was surrounded by a bold-line and selecting inside the second rectangle highlights the top surface of the block, excluding the area of the first rectangle.
Why is the second rectangle treated differently than the first (bold border, no interior select); is this a SketchUp flaw, or something I’m not understanding about SketchUp construction?
See Attched Model.
Rectangles.skp (11.7 KB)

I can’t see your model (no SketchUp available on this PC now) but drawing any new loop of edges on an existing face (like your rectangles are) with only one common vertex with the outer faceś loop will sort of confuse sketchUp. Selecting faces proves that, the smaller loop doesn’t split the larger face into two new separate smaller faces like it should. I guess that is what you are doing, for the second rectangle only has one vertex in comon. It is now surrounded by thick profiles indicating that the rectangle has its own face supperimposed on a face ath the same location.

Select the small face with thick profils, right click on it and select ‘Reverse faces’ in the context menu.
Now you’ll see Z-fighting (two faces that fight for being rendered on screen) when you orbit.

It looks like, as you’ve said, I confused SketchUp. I was experimenting with a simple procedure to position a rectangle corner at an exact position on the surface of the block. Essentially I was using the first rectangle as construction-lines to position the second rectangle. I would then erase the two lines of the first rectangle, having positioned the second indexed off its lower right corner. I’m sure there must be some better way to position the rectangle.

Seems like the Tape-Measure tool can create the construction lines.

Thanks for your help.

Construction lines do help, yes.
Don’t overdo using them. I consider them for temporary use if I ever use them. And delete them as soon as possible.

You can create a new endpoint drawing an edge on top of a larger edge.
Same thing on an edge perpendicular to the first large edge.
You’ll now have two new endpoints to reference to, when creating the small rectangle.
(Many ways to skin a …)

Maybe you should (if not yet done) create a style and scene with ‘Endpoints’ ON and with size 3 or more. (Menu Window > Style > Edit > Edge Settings (first icon) > check ‘Endpoints’ with size 3
Create a scene with these changes updated.