Two root causes made justified text overrun the window's right edge
in real user sessions (every line ending in the "$" truncation
marker), while emacs -Q looked fine:
1. Windows WITHOUT fringes draw the truncation/continuation
indicator in the text area's LAST COLUMN, so the usable width is
one character less than window-body-width. ekp reserved only a
2px margin: with fringes disabled (a common minimal setup, and
all ttys) every line that hit the target width exactly had its
final glyph displaced by the "$". ekp-region--window-pixel now
reserves one frame-char-width when the window has no right
fringe. (The same lesson ebox-playground encodes in its
viewport-width reserve.)
2. Measurement was blind to the buffer's display context:
string-pixel-width works in a bare hidden buffer, ignoring
face-remapping-alist — which is where text-scale-mode, themes
and per-buffer font tweaks live. Under a remap, the DP laid
lines out with one font's metrics and the display rendered them
with another's: scale +3 made 5 of 7 sample lines overflow a
1330px window by up to 900px (GUI-measured). Measurement now
runs with the destination buffer's face-remapping-alist (the
29/30-compatible equivalent of Emacs 31's string-pixel-width
BUFFER argument), and the width/paragraph caches key on that
context so buffers at different scales never alias. text-scale
changes also trigger a re-flow in ekp-auto-justify-mode.
Ground truth, measured with window-text-pixel-size in GUI Emacs
across 7 display contexts (plain / text-scale ±| face remap /
no-fringes / no-fringes+scale / narrow+scale): the widest justified
line equals the target width exactly in every case, zero lines
overflow. Verbatim code blocks are exempt by design (they never
reflow, like any code line in a narrow window).
New tooling so this never regresses invisibly:
- M-x ekp-diagnose: renders a probe line in YOUR buffer and reports
target vs rendered width — run it in any session where justified
text looks wrong.
- tests/ekp-gui-verify.el: M-x ekp-gui-verify (single check in a
customized session) and ekp-gui-verify-matrix (the 7-case table,
for emacs -Q).
- 2 new batch ERT tests pin the context-keyed caches (94 total).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
