Resume-at-line treats non-modal G53 as the active work coordinate system

[righteousgambit]

Summary

Resume-at-line currently considers G53 alongside G54 through G59.3 when it reconstructs the active work coordinate system. It then replays whichever command appeared most recently.

G53 is a non-modal qualifier for a machine-coordinate move, not a persistent work-coordinate-system selection. Replaying a bare G53 therefore does not restore the interrupted program's active WCS.

The behavior is present on develop at 2ea66c10e96d88013a04441134d3d4f8e8c88510: WCS recovery logic. LinuxCNC documents G53 as non-modal and G54 through G59.3 as WCS selectors: coordinate-system documentation. The Community Firmware likewise consumes G53 as a machine-coordinate flag for the associated movement: firmware implementation.

Minimal reproduction

G55
G53 G0 Z-2
G1 X10 F100
G1 X20

Preview a resume at line 4.

Expected:

• The recovery command sequence contains buffer G55.
• The controller's intentional safe-Z command remains buffer G53 G0 Z-2.

Actual:

• The sequence contains a bare buffer G53 instead of restoring G55.
• The separate safe-Z command is also present.

Impact

If the machine still retains G55, this may not change the resulting position. If the retained WCS differs after a reset, reconnect, or manual WCS change, however, resume positioning can occur in the wrong coordinate frame. Resume should reconstruct the file's modal WCS deterministically rather than depend on retained machine state.

Acceptance criteria

• Only G54 through G59.3 participate in modal WCS recovery.
• The intentional safe-Z move still uses G53 G0 Z-2.
• A regression test proves that a later G53 move does not replace an earlier active WCS.