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I'd like to retrieve part of shape A from layer A in relation to the boundary of my layer B. After making an intersection, I end up with a new layer, "intersect_layer".

Using spatial selection with my "intersect_layer" layer and my B layer, with the "is inside" option, intersected shape A is not selected.

I've tried cutting and separating multipolygons, with the same result using QGIS 3.36.1

In the image, I'm trying to recover the purple shapes on top of the green shape. However, since the purple shape at the bottom of the image was originally not just "inside", QGIS doesn't select it for me (whereas now it's an entity in its own).

Conversely, some shapes are not selected because a small piece was on the outside before the intersection.

I suspect that, since the intersection was made with the green shape and I'm trying to select what's inside, the lines are perfectly superimposed and therefore don't count as "inside" (because it is touching the edge is what i'm trying to say).

EDIT answering @Erik : adding "touches" and/or "crosses"

Now QGIS select outside-touching shapes

I'd like to retrieve part of shape A from layer A in relation to the boundary of my layer B. After making an intersection, I end up with a new layer, "intersect_layer".

Using spatial selection with my "intersect_layer" layer and my B layer, with the "is inside" option, intersected shape A is not selected.

I've tried cutting and separating multipolygons, with the same result using QGIS 3.36.1

In the image, I'm trying to recover the purple shapes on top of the green shape. However, since the purple shape at the bottom of the image was originally not just "inside", QGIS doesn't select it for me (whereas now it's an entity in its own).

Conversely, some shapes are not selected because a small piece was on the outside before the intersection.

I suspect that, since the intersection was made with the green shape and I'm trying to select what's inside, the lines are perfectly superimposed and therefore don't count as "inside" (because it is touching the edge is what i'm trying to say).

EDIT answering @Erik : adding "touches" and/or "crosses"

Now QGIS select outside-touching shapes

Edit 2 : testing all the options of "Select by Location"

touch, contain, equal, cross : no selection

Overlap, disjoint, intersect gives me differents results, none that answer (within is stated above).

It could the layer that is cut in a bad way (tried intersect, intersect + cut, intersect + multipart to singleparts) but at this point I do not know what to try.

I'd like to retrieve part of shape A from layer A in relation to the boundary of my layer B. After making an intersection, I end up with a new layer, "intersect_layer".

Using spatial selection with my "intersect_layer" layer and my B layer, with the "is inside" option, intersected shape A is not selected.

I've tried cutting and separating multipolygons, with the same result using QGIS 3.36.1

In the image, I'm trying to recover the purple shapes on top of the green shape. However, since the purple shape at the bottom of the image was originally not just "inside", QGIS doesn't select it for me (whereas now it's an entity in its own).

Conversely, some shapes are not selected because a small piece was on the outside before the intersection.

I suspect that, since the intersection was made with the green shape and I'm trying to select what's inside, the lines are perfectly superimposed and therefore don't count as "inside" (because it is touching the edge is what i'm trying to say).

EDIT answering @Erik

Now QGIS select outside-touching shapes

I'd like to retrieve part of shape A from layer A in relation to the boundary of my layer B. After making an intersection, I end up with a new layer, "intersect_layer".

Using spatial selection with my "intersect_layer" layer and my B layer, with the "is inside" option, intersected shape A is not selected.

I've tried cutting and separating multipolygons, with the same result using QGIS 3.36.1

In the image, I'm trying to recover the purple shapes on top of the green shape. However, since the purple shape at the bottom of the image was originally not just "inside", QGIS doesn't select it for me (whereas now it's an entity in its own).

Conversely, some shapes are not selected because a small piece was on the outside before the intersection.

I suspect that, since the intersection was made with the green shape and I'm trying to select what's inside, the lines are perfectly superimposed and therefore don't count as "inside" (because it is touching the edge is what i'm trying to say).

EDIT answering @Erik : adding "touches" and/or "crosses"

Now QGIS select outside-touching shapes