Snap Grid + draw Line causes first pixel to be missed when drawing toward top/left, last pixel to be missed when drawing toward bottom/right about aseprite HOT 1 OPEN

Thanks for this report! I will take this opportunity to summarize some related problems.
Snap to Grid is sometimes confusing dependant on:

1. Grid size VS pixel size.
2. Different behaviors according tool used.
3. Truncation issue when the end coordinate > start coordinate (bug).

1: Pixel size is 1 and Grid cell is 3x3, the brush snaps to the closest grid cell origin:

It looks OK.
But when the pixel size is 6 and the grid cell is 3x3, the brush snaps its top-left corner with the origin of the grid cell closest to the cursor position (i.e. the center coordinate of the brush) , this is a bug.

Related: https://community.aseprite.org/t/improvement-snap-to-grid/4710

2: Different behaviors: Pencil tool VS Rectangle tool

When Snap to Grid is ON, most tools work like they do on the left side (snapping to the origin of each grid cell).
But others work like on the right side (snapping to corners within a grid cell or a group of grid cells), these tools are the Line tool, Rectangle tools, and Ellipse tools.
Depending on the needs of the artist, one behavior or another will be better. I think it would be nice to have some checkbox to select it.
Related: #4190

3: Exactly the problem you described above with the Line tool when the end coordinate is greater than the start coordinate.

from aseprite.