Different behaviour of arrows with the same preset: before and after the mouse leaves the board

[lluisalemanypuig]

Exact URL of where the bug happened

I've documented the bug in https://lichess.org/analysis but it also works in puzzles and games. I suspect it also happens in everything that involves the same component used for the board in "/analysis"

I've tried this on

• Firefox: Version 150.0.2
• Chromium: Version 147.0.7727.116

Steps to reproduce the bug

The bug is easy to reproduce (I showcase this here: https://youtu.be/th-BdRyn7Dw):

1. Set your settings so that arrows can be drawn only "valid moves".
   Important Note The valid moves option has nothing to do with actual valid moves: it is only intended to snap on squares diagonally, horizontally, vertically, or in a knight move. For the sake of brevity, I'll call these squares DHVK.
2. Go to analysis (or open an actual game)
3. Try to draw a line to a square where the arrow does not snap -- you shouldn't be able to as per the settings applied in step 1
4. Now, hold down the appropriate mouse button for drawing arrows, and move the cursor out of the board area; do not stop holding the button.
5. Move the cursor back to the board area: the arrows you can draw snap to all squares you move the cursor to.

What did you expect to happen?

The arrows not snapping to DHVK squares. If the arrows only snap to DHVK squares (as per the settings), the arrows should still only snap to DHVK even after the cursor has left the board and re-entered.

What happened instead?

After the cursor leaves the board, the arrows snap to all squares: only DHVK squares should be allowed.

Operating system

Ubuntu 24.04

Browser and version (or alternate access method)

Firefox 150.0.2

Additional information

I've tried this on both

• Firefox: Version 150.0.2
• Chromium: Version 147.0.7727.116

[nicvagn]

When I had snap to valid move on it snapped to all these locations:

[nicvagn]

I believe this is an chess ground issue

[lluisalemanypuig]

I never intended to limit the "valid move" concept to just pawns. I didn't even pay attention to the fact that I was using a pawn. I was trying to draw attention to the fact that there is a difference in behaviour after the mouse has moved out of the board while holding the button.

[lluisalemanypuig]

I believe this is an chess ground issue

If this is not related to lichess at all this bug report shouldn't be here, sorry.

[lluisalemanypuig]

When I had snap to valid move on it snapped to all these locations:

Doesn't do that for me. I get the right behaviour even when I start in an empty square

[nicvagn]

I can cause the issue with a minimal chessground.

This is an issue, but it is an issue for the chess grounds git repo: lichess-org/chessground

So chessground is the board UI, and I can get:

This is not a valid move, and snap to legal move is set.

[lluisalemanypuig]

This is not a valid move, and snap to legal move is set.

If I get the right behaviour, but you don't, maybe the issue is worth taking a look at. I don't know, I posted this because there is a mismatch between expectation and result.

This is an issue, but it is an issue for the chess grounds git repo: lichess-org/chessground

Then the issue should be moved. Sorry I didn't look deep enough to post it there directly.

[nicvagn]

You are good friend, I am just trying to help out. I lacked a solid understanding of what you where saying, and through discourse I think I understand better.

I think this happens because chessgrounds is just the ui, and is ignorant of the rules of chess. This is great for flexibility.

Chessgrounds uses geometry to discover legal moves, not calculation. For example if two coordinates x1,y1 and x2,y2 meet
diff(x1, x2) * diff(y1, y2) === 2
this is included in the snap "field"
There are others for all the pieces, and something smells bad.

[johndoknjas]

Are we sure this isn't just intended behaviour? It provides users with a way to draw arrows anywhere, even if most of the time they only want valid arrows.

[nicvagn]

But, it doesn't really let them draw arrows anywhere. It still snaps to squares. Just squares that don't represent valid moves.

[lluisalemanypuig]

I have changed the description of the PR because the "valid moves" concept is really getting in the way.

[lluisalemanypuig]

Are we sure this isn't just intended behaviour? It provides users with a way to draw arrows anywhere, even if most of the time they only want valid arrows.

Could be the case that this is intended behaviour. I honestly don't know that myself.