Quick answer: GameMaker tile collision missing thin slope edges? Default tile collision uses bounding-box; slopes need per-pixel collision shapes.
Player walking down a slope: occasionally pops through to the layer below.
Per-tile collision shapes
In tile set editor, define per-tile shape. Diagonal-edge tiles get triangle collision.
Or use a separate collision layer
Visual tiles for rendering; collision objects for the slope. Decoupled; precise.
Snap player to surface
Per-frame, sample ground height; clamp player Y. Manual; works for any slope shape.
“Tile collision is bounding-box. Sloped geometry needs custom shapes.”
If your game has slopes, the per-tile collision setup is one-time work. Plan the tileset accordingly.