Set the pending-wrap flag only at the phantom column - #115
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SetCursorRaw set PendingWrap = true on EVERY raw advance -- after every printed character -- documented as harmless because "only the boundary column reads it". tomlm#93 quietly broke that contract by adding two readers that consult the flag anywhere: SettleForEditing (ICH/DCH/ECH) and the CUB phantom-column adjustment. From then on, any CUB or DCH issued immediately after printing acted one column LEFT of the cursor. On screen that was the asciiquarium bug: duplicated fragments trailing left-moving sprites (CUB off by one leaves the old cells unerased) and characters eaten from right-moving ones (DCH deletes one cell early). It looked rate- and machine-dependent, but it never was -- it needed a print immediately followed by a relative-back or edit in the stream, which curses emits as a cursor-motion optimisation, so it tracked what the animation happened to draw. A 16-byte repro settles it: ESC[5;1H AB ESC[1P eats the B. The fix makes the flag state a fact instead of trusting its readers: PendingWrap = x > _scrollRight, true exactly at the one-past-the-margin position only a fill can produce. DECRC's explicit restore still runs through SetPendingWrap afterwards, so the saved ambiguity survives. Tests: the three repros (DCH after print, CUB after print, a stale flag carried across a cursor address), plus two guards proving the fix does not regress what the flag is FOR -- autowrap at the last column, and SettleForEditing's fill-then-insert case. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
tomlm
approved these changes
Aug 30, 2026
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Fixes the asciiquarium rendering corruption: duplicated fragments trailing left-moving sprites, missing characters on right-moving ones.
Root cause
SetCursorRawsetPendingWrap = trueon every raw advance — after every printed character — documented as harmless because "only the boundary column reads it." #93 broke that contract by adding two readers that consult the flag anywhere:SettleForEditing(ICH/DCH/ECH) and the CUB phantom-column adjustment. From then on, any CUB or DCH issued immediately after printing acted one column left of the cursor.It looked rate- and machine-dependent, but never was: it needs a print immediately followed by a relative-back or edit in the stream, which curses emits as a cursor-motion optimisation — so it tracked what the animation happened to draw. A 16-byte repro settles it:
Fix
The flag now states a fact instead of trusting its readers:
PendingWrap = x > _scrollRight, true exactly at the one-past-the-margin position only a fill can produce. DECRC's explicit restore still runs throughSetPendingWrapafterwards, so the saved ambiguity survives.Verification
Co-Authored-By: Claude Fable 5 noreply@anthropic.com