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tty: the grid was 80x24 on a 160x50 screen, and wrapping was eager

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Three symptoms, two causes, both in the terminal layer.

**The grid and TIOCGWINSZ disagreed about the size.** vt_init() sizes every
grid from gfx_cols()/gfx_rows(), and it runs at main.c:75 -- while the display
driver attaches its scanout from the PCI bus walk at main.c:99. So the grids
were the 80x24 fallback, and TIOCGWINSZ, which computed its own answer live,
said 160x50.

That one disagreement produced all three things visible in the report: text
stopped two thirds of the way down the screen, `ls` laid out 160 columns into
an 80-column grid so words split across the wrap ("vers" / "ion"), and zsh's
cursor tracking was wrong on every line.

Fixed in two places, because either alone would leave the trap set:
gfx_attach_scanout() now calls vt_resize(), so the grids learn the real screen
the moment there is one; and TIOCGWINSZ reports v->cols/v->rows rather than
measuring again. One number, from the terminal being asked.

**Wrapping was eager.** A glyph printed in the last column moved the cursor to
the next line immediately. Real terminals set a pending-wrap flag and wrap when
the *next* glyph arrives -- which is why a line exactly as wide as the screen
takes one row and not two, and why a program can write the bottom-right cell
without scrolling.

zsh's partial-line marker depends on that rule directly: it writes exactly
COLUMNS-1 spaces and expects to still be on the line. With eager wrap it left a
reverse-video '#' above every prompt, which is the hashtag in the report. Both
are gone.

Verified by screendump: `ls /proc` now fits one line with "version" intact, no
markers, and output fills the screen to the last row. schfs-check passes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2
This commit is contained in:
Nexoniarz 2026-09-13 12:49:13 +02:00
commit a5b0d5a0ef
5 changed files with 137 additions and 25 deletions
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7
include/vt.h
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@ -79,6 +79,12 @@ struct vt {
struct vtcell *cells; // cols*rows, row-major, into vt_storage
int cx, cy; // cursor, in cells
// Deferred wrap, the rule every real terminal follows and none of them
// document well: a glyph printed in the last column does NOT move the
// cursor to the next line. It sets this instead, and the wrap happens when
// the *next* glyph arrives. Anything that moves the cursor deliberately --
// CR, LF, backspace, an escape sequence -- clears it.
int wrap_pending;
int saved_cx, saved_cy; // DECSC/DECRC
uint8 fg, bg; // current SGR colours
@ -118,6 +124,7 @@ struct vt {
extern int vt_active;
void vt_init(void);
void vt_resize(void); // a scanout arrived; re-measure
struct vt *vt_get(int i); // 0-based, clamped, never null
struct vt *vt_current(void);
int vt_switch(int i); // 0-based; repaints. -1 if out of range