diff --git a/docs/claude/probe-readings.md b/docs/claude/probe-readings.md
index 9e3e236..f0e92ec 100644
--- a/docs/claude/probe-readings.md
+++ b/docs/claude/probe-readings.md
@@ -312,6 +312,9 @@ Two lines of the self-check output are assertions in disguise:
   — that is how it tells a gesture from its own echo — so a fit that wrote the group alone reads
   back as the zoomed range one frame later.
 - `[layout-probe] <action> rows=N buttons=B done=D done_box=[x,y,WxH] rows_y=[..] close=[..] add=[..] traces=[..]`
+  `side_w` is the sidebar column's width and **must be its own request (320), never 0**: it read 0 while
+  the divider sat at the window's right edge, which is a panel that is not there. It was 320 during the
+  first frames and 0 once the window had settled, so read it from a `placed` line rather than the first.
   (stderr, `UDPSCOPE_PROBE_LAYOUT_MS`) — **`buttons` must be `2N+1`** while the editor is
   up (one `×` per row, one `+` per boundary) and **0 on the `editor off` line**: the
   buttons exist only while the editor does. `done` follows the same rule for the editor's
diff --git a/src/ui_probe.cpp b/src/ui_probe.cpp
index 9a53eba..64c1cfe 100644
--- a/src/ui_probe.cpp
+++ b/src/ui_probe.cpp
@@ -1536,11 +1536,15 @@ void layout_probe_report(App& a, const char* what) {
     }
   }
   std::fprintf(stderr, "[layout-probe] %-12s rows=%d buttons=%zu done=%d done_box=[%s] "
-                       "rows_y=[%s] close=[%s] add=[%s] resize=[%s] traces=[%s]\n",
+                       "rows_y=[%s] close=[%s] add=[%s] resize=[%s] traces=[%s] side_w=%d\n",
                what, rows, a.layout_buttons.size(), a.layout_done_button != nullptr ? 1 : 0,
                done_box.c_str(), geom.c_str(), close_tops.c_str(), add_tops.c_str(),
                resize_tops.c_str(),
-               traces.c_str());
+               traces.c_str(),
+               // The sidebar, which is the pane a divider position can silently take away: `side_w`
+               // must be its own request (320) rather than 0, and `divider` the position the window's
+               // width less the sidebar's column left it at.
+               a.side_box != nullptr ? gtk_widget_get_width(a.side_box) : -1);
   std::fflush(stderr);
 }