Improve comments from #3822 (#3836)

### What
Address some PR concerns from:
 -  #3822

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested [demo.rerun.io](https://demo.rerun.io/pr/3836) (if
applicable)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG

- [PR Build Summary](https://build.rerun.io/pr/3836)
- [Docs
preview](https://rerun.io/preview/9926b13d4f8bc925e37b506f46540058df211249/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/9926b13d4f8bc925e37b506f46540058df211249/examples)
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- [Recent benchmark results](https://ref.rerun.io/dev/bench/)
- [Wasm size tracking](https://ref.rerun.io/dev/sizes/)

crates/re_space_view_spatial/src/parts/boxes2d.rs

@@ -190,7 +190,10 @@ impl ViewPartSystem for Boxes2DPart {
             return false;
         }
 
-        // If this is a 3D view and there's no parent pinhole, do not include this part.
+        // 2D parts can only ever be rendered properly as part of a 3D scene if the
+        // transform graph includes a pinhole projection. Pinholes only map from 2D children
+        // to 3D parents, so if the required pinhole exists, it must be an ancestor.
+        // Filter them otherwise to avoid ending up with 2D content mixed into 3D scenes.
         if ctx.class == "3D" && !ctx.has_ancestor_pinhole {
             return false;
         }

crates/re_space_view_spatial/src/parts/images.rs

@@ -689,7 +689,10 @@ impl ViewPartSystem for ImagesPart {
             return false;
         }
 
-        // If this is a 3D view and there's no parent pinhole, do not include this part.
+        // 2D parts can only ever be rendered properly as part of a 3D scene if the
+        // transform graph includes a pinhole projection. Pinholes only map from 2D children
+        // to 3D parents, so if the required pinhole exists, it must be an ancestor.
+        // Filter them otherwise to avoid ending up with 2D content mixed into 3D scenes.
         if ctx.class == "3D" && !ctx.has_ancestor_pinhole {
             return false;
         }

crates/re_space_view_spatial/src/parts/lines2d.rs

@@ -185,7 +185,10 @@ impl ViewPartSystem for Lines2DPart {
             return false;
         }
 
-        // If this is a 3D view and there's no parent pinhole, do not include this part.
+        // 2D parts can only ever be rendered properly as part of a 3D scene if the
+        // transform graph includes a pinhole projection. Pinholes only map from 2D children
+        // to 3D parents, so if the required pinhole exists, it must be an ancestor.
+        // Filter them otherwise to avoid ending up with 2D content mixed into 3D scenes.
         if ctx.class == "3D" && !ctx.has_ancestor_pinhole {
             return false;
         }

crates/re_space_view_spatial/src/parts/points2d.rs

@@ -212,7 +212,10 @@ impl ViewPartSystem for Points2DPart {
             return false;
         }
 
-        // If this is a 3D view and there's no parent pinhole, do not include this part.
+        // 2D parts can only ever be rendered properly as part of a 3D scene if the
+        // transform graph includes a pinhole projection. Pinholes only map from 2D children
+        // to 3D parents, so if the required pinhole exists, it must be an ancestor.
+        // Filter them otherwise to avoid ending up with 2D content mixed into 3D scenes.
         if ctx.class == "3D" && !ctx.has_ancestor_pinhole {
             return false;
         }

crates/re_space_view_spatial/src/space_view_2d.rs

@@ -80,9 +80,16 @@ impl SpaceViewClass for SpatialSpaceView2D {
         );
 
         // If this is the root space view, and it contains a part that would
-        // prefer to be 3D, don't spawn the 2D view. This is because it's never
-        // possible to correctly project 3d objects to a root 2d view since the
-        // the pinhole would go past the root.
+        // prefer to be 3D, don't spawn the 2D view.
+        //
+        // Since pinhole projections provide a mapping between a 2D child space and a 3D
+        // parent space, it means that for any 3D content to to be projected into a 2D space,
+        // there must exist a common 3D ancestor-space which has a *child* which is a 2D space
+        // (and contains a pinhole.) But if this space origin is at the root itself, that common
+        // ancestor would need to be a parent of the root, which doesn't exist. As such, it's
+        // impossible that this space would be able to correctly account for the required
+        // content. By not spawning a 2D space in this case we ensure the 3D version gets chosen
+        // by the heuristics instead.
         if space_origin.is_root() {
             let parts = ctx
                 .space_view_class_registry