docs: the readouts strip

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/claude/invariants-plot-and-layout.md

@@ -148,6 +148,18 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   characters, which made the readouts' minimum width 304 px; that pushed the sidebar to 304 px
   (`dock_w=304`) and defeated the 280 pin. Widening any cell is the same
   trap -- read `[readouts] ... dock_w=` (must be 280) and the `dock cursors:` / `dock measures:` `fits=`, and for the Events block `dock events: ... dock_w=` (268).
+- **The readouts strip is a fixed 26 px, scrolls sideways, and is shown only without the dock.**
+  `readouts_strip` (a `GtkScrolledWindow`, `height-request` 26, horizontal `external`, vertical `never`) sits
+  in `left_column` between `plot_overlay` and `badge_bar`. Its height is a request and not a measurement of
+  its chips (the buttons are trimmed by `button.udpscope-strip-button` in `ui_widgets.cpp`, or Adwaita's
+  34 px flat button would push it past 26), the chips' cells are fixed-width ellipsized labels like the
+  dock's, and an overflow scrolls with no bar taking height: "layout must not depend on content" for the
+  strip, the same as for the badge bar. It is *visible* exactly when `show-sidebar` is false
+  (`sync_readouts_strip()`, from the tick after the wish is re-asserted and before `set_collapsed()` --
+  read after the collapse it would show for a frame each time the window narrowed), and hidden when the dock
+  is on screen, overlaid included, so the plot's 26 px go to one pane of readouts and never both. The plot
+  is 26 px shorter while it shows; that is the dock's state moving the layout, not content. Reading
+  `h=26` with `visible=1` (and `h=0` with `visible=0`) in `strip:` is the assertion.
 - **Trace slots are identified by signal name, not channel index.** Enabling a signal
   or a new CONFIG renumbers the channel list, so an index would silently re-point a
   trace at another signal.