Jump-mode backspace past beginning of move queue locks game

[WeaverThree]

Environment

Megamek 0.49.18

Tested under Adoptium Java 11 and 17 on Windows 10, and OpenJDK 11 and Adoptum 17 on current Fedora Linux, problem persists in all cases.

Description

When you're in movement phase with a mek and in jump mode and backspace past the beginning of the movement list, the game completely freezes and needs to be task killed to close. This can be done just be pressing the 'jump' button and backspace once.

Files

megamek.log - doesn't appear to have anything from this problem
jump-backspace-to-crash-0.49.18.sav.gz - playername Weaver - this is just a pxh-1 by itself ready to move on turn one, so select 'jump' and hit backspace to crash it.

[kuronekochomusuke]

looks like it gets into an infinite loop here in MovementDisplay

        // Keep removing last step until it's a valid movement step.
        while (cmd.getLastStepMovementType() == EntityMovementType.MOVE_ILLEGAL) {
            removeLastStep();
            removed = true;
        }

[Lu9us]

It looks like this is due to an issue in MovementDisplay.removeIllegalSteps() as stated as it treats the start of the jump as Illegal since you can't end a jump without moving then when it is in the MovementDisplay.removeLastStep() this will call cmd.removeLastStep() which clears cmd's steps as expected but when MovementDisplay.clear() is called it will re-add a START_JUMP step because the entity is in jump mode which is once again deemed illegal causing an infinite loop.

Theres some extra steps that I don't quite have a full handle on yet but this is the gist of the issue.

[gsparks3]

Interestingly, that makes this related to #4970 - jumping into your starting hex shouldn't be illegal for non-infantry units. Although it would still be illegal for infantry, so you can't just use that as the fix instead, have to do both.

[Lu9us]

So this is pretty explictily excluded in MoveStep.isLegalEndPos:

... 
else if (isJumping() && (distance == 0)) {
            // Can't jump zero hexes.
            legal = false;
}....

Not sure if the rules have changed since this code was written or I'm missing something.

[HoneySkull]

Thank you for the bug report @WeaverThree and for the fix @Lu9us. I definitely did not anticipate removing move steps would also create move steps in the case of jump mode. I feel like the fix by @Lu9us is consistent with the intent of the "undo" nature of the backspace key.