diff --git a/docs/ARCHITECTURE.md b/docs/ARCHITECTURE.md
index d0c01a0..eb4586b 100644
--- a/docs/ARCHITECTURE.md
+++ b/docs/ARCHITECTURE.md
@@ -71,6 +71,12 @@ clipboard writes are disabled by default. The renderer never imports the
 upstream terminal type, and protocol replies return through the same actor input
 queue as human input. See decision 0005.
 
+Terminal cell size is measured from the text system rather than assumed, and the
+viewport's pixel extent is reported to the PTY. Both were wrong before: the
+advance was a hardcoded guess 13% wider than the face actually painted, and
+`ws_xpixel`/`ws_ypixel` were zero, so every program doing pixel arithmetic was
+told the window had no size.
+
 Each `lumbridge-runtime` actor owns one `PtySession` on a dedicated thread.
 Bounded command and event queues serialize input, resize, close, and shutdown
 against ordered raw-byte output. A generic pane-indexed registry now owns and
diff --git a/docs/decisions/0009-responsive-ultrawide-work-lanes.md b/docs/decisions/0009-responsive-ultrawide-work-lanes.md
index ada05d3..f691a91 100644
--- a/docs/decisions/0009-responsive-ultrawide-work-lanes.md
+++ b/docs/decisions/0009-responsive-ultrawide-work-lanes.md
@@ -17,8 +17,14 @@ independently of selection.
 Terminal geometry is computed from the width of one panel and the height of its
 middle 60% region, including gaps and content insets. The selected real PTY is
 resized through the runtime actor when the responsive panel count or window
-bounds change. On the 3440×1440 reference display the current fixed-cell spike
-derives approximately 71 columns by 42 rows per panel.
+bounds change. Cell size is **measured**, not assumed: the advance of `0` in the terminal face
+comes from the text system, so the column arithmetic and the painted glyph agree
+by construction. This paragraph previously quoted "approximately 71 columns by
+42 rows per panel", derived from a hardcoded 8.4 px advance that nothing had
+ever measured. The real advance is about 7.3 px, so those numbers were roughly
+13% short — one full-width pane on the reference display measures 140 columns,
+not 122. Any figure quoted here is a consequence of the measurement and the
+current panel count, and will move when either does.
 
 The runtime slice now owns three independent real PTYs through a pane-indexed
 session registry, one for every terminal fixture. The Markdown, browser, and