@@ -1887,6 +1887,16 @@ if(PYTHON3_EXECUTABLE)
            COMMAND ${PYTHON3_EXECUTABLE}
                    ${CMAKE_CURRENT_SOURCE_DIR}/tests/look/contrast.py)
 
+  # Text in a window that has lost focus keeps its contrast (GTK :backdrop),
+  # and disabled text still looks disabled (#210).  Compiles
+  # tests/look/backdrop.C against this build; SKIPs without Xvfb or PIL.
+  add_test(NAME look_backdrop
+           COMMAND ${PYTHON3_EXECUTABLE}
+                   ${CMAKE_CURRENT_SOURCE_DIR}/tests/look/login_backdrop.py
+                   backdrop ${CMAKE_BINARY_DIR}/look-backdrop
+                   --libdir ${CMAKE_BINARY_DIR} --tag after)
+  set_tests_properties(look_backdrop PROPERTIES SKIP_RETURN_CODE 77 TIMEOUT 300)
+
   # The offscreen renderer's .rgb output against ImageMagick and PIL (#231).
   # Needs `ninja render-rgb`, Xvfb, convert and PIL; SKIPs without them.
   add_test(NAME look_imageconv
 
@@ -0,0 +1,36 @@
+---
+type: map
+title: "An atom's shell with a non-bonding orbital is drawn at that orbital (#140)"
+area: mo-diagram
+section: "The MO correlation diagram (#132)"
+paths: ["src/tdat/chemistry/MoDiagram.C", "include/tdat/MoDiagram.H"]
+issues: [140, 132]
+---
+`placeFragments()` draws every component of an atom's shell on one row.
+The row is the mean of the orbitals the components became, except when
+the shell has a non-bonding orbital: then the row is at that orbital's
+energy, so it sits level with its parent (water's 1b1 on O 2p).
+
+Non-bonding means `nonbondingShellFraction() >= NONBONDING_SHELL_SHARE`
+(0.99): the share on that one shell over the share on everything the
+diagram draws. Polarisation functions of column atoms (O d, H p) are
+left out of the denominator, because without them symmetry-forbidden
+mixing gives exactly 1 in any basis; atoms in neither column count
+against. Because of that, every b1 orbital of water is "pure O p",
+including diffuse virtuals, so only the lowest qualifying orbital per
+irrep pins the row.
+
+A lone pair from s/p mixing (water 3a1, NH3 3a1) is not single-shell
+(f = 0.76 and 0.86 in ORCA def2-SVP; 0.93 and 0.94 in extended Huckel)
+and is drawn in the usual way, connected to s, p and the ligand set, as
+in Albright-Burdett-Whangbo (2013) Figs. 7.2 and 9.4. classify() tags it
+MIXED, not nb, when its irrep is on both sides and it holds at least
+`LINK_SHARE` (0.05) of each column's drawn shells; with a composition,
+connect() draws every side at or above `LINK_SHARE`, nb or not. Before
+this, water 3a1 was tagged nb by the count and drawn to O only.
+
+The panel prints the fraction per level and shell as `[MOSHELL]` under
+`ECCE_DEBUG_MOSYM`. The canvas opens the molecular column out too (it
+calls `columnGeometry()` with spreading on for it), so a pinned row and
+its orbital can still be drawn a few pixels apart when other molecular
+levels crowd it: 4 px in extended Huckel water at 1400x950.
@@ -1,12 +1,13 @@
 ---
 type: map
-title: "Fragment columns may be opened out for legibility; the molecular column never is"
+title: "Fragment columns may be opened out for legibility, and so is the molecular column"
 area: mo-diagram
 section: "The MO correlation diagram (#132)"
 paths: ["src/apps/builder/MoDiagramCanvas.H"]
 issues: [132]
 ---
-Fragment columns may be opened out for legibility; the molecular
-column never is, because its energies are the measurement. Whatever
+Fragment columns may be opened out for legibility, and so is the
+molecular column (the canvas spreads it too), so a crowded molecular
+level can be drawn a few pixels off its energy. Whatever
 computes a correlation line's endpoint must use the same `columnGeometry()`
 answer the levels are drawn with, or the lines point at nothing.
@@ -30,5 +30,6 @@ MOPAC → spec) is the bridge it would reuse.
 - [Look at `tools/modiagram/render`'s output before believing any layout claim](look-at-tools-modiagram-renders-output-before.md)
 - [Without `symops` every fragment column is empty and no line is drawn](empty-and-no-line-is-drawn.md)
 - [A fragment column is not sorted by energy](not-sorted-by-energy.md)
-- [Fragment columns may be opened out for legibility; the molecular column never is](fragment-columns-may-be-opened-out-for.md)
+- [Fragment columns may be opened out for legibility, and so is the molecular column](fragment-columns-may-be-opened-out-for.md)
 - [MO composition: MoAoBasis, Mulliken by default (#161)](mo-composition-is-moaobasis-mulliken-by-default.md)
+- [An atom's shell with a non-bonding orbital is drawn at that orbital (#140)](a-shell-with-a-non-bonding-orbital-is-drawn-at-it.md)
@@ -0,0 +1,22 @@
+---
+type: pitfall
+title: "GTK :backdrop fades every label to ~2.8:1; ewxApp::applyBackdropStyle() undoes it, and headless runs need ECCE_TEST_BACKDROP"
+area: wx-viewer
+paths: ["src/wxgui/ewxClasses/ewxApp.C", "tests/look/login_backdrop.py", "tests/look/backdrop.C", "tests/look/organizer_backdrop.py"]
+issues: [210]
+---
+Adwaita draws text in an unfocused toplevel (`:backdrop`) in a 50 % mix of
+text and background: a normal label drops from 11.6:1 to 2.8:1, so with
+several ECCE windows open the unfocused ones look disabled.
+`ewxApp::applyBackdropStyle()` (called from `OnInit`) adds an
+application-priority CSS sheet giving `label`, `entry` and `.view` the
+theme's normal colour in `:backdrop` and `@insensitive_fg_color` for
+`:disabled:backdrop`. GTK3 CSS has no `:not()`, so the disabled case is its
+own rule. `ECCE_BACKDROP=theme` switches the fix off.
+- Without a window manager (Xvfb) GTK never enters `:backdrop`: every
+  window stays active whatever `xdotool windowfocus` does. Tests set the
+  flag themselves (`backdrop.C`) or run the app with `ECCE_TEST_BACKDROP=1`,
+  which re-asserts it on every top-level window each 200 ms.
+- `ctest -R look_backdrop` measures the ink contrast of a window in
+  `:backdrop` against an active one; it fails below 85 %, or if a
+  disabled label is no dimmer than a normal one.
@@ -11,6 +11,7 @@ Read this before changing any dialog, panel, sizer, grid, ewx control or the Ope
 
 ### wxWidgets 3.2/GTK3, the 3D viewer, and C++ pitfalls
 
+- [Every plot is an `ewxPlotCtrl` styled by `PlotStyle.H`, the palette the vibrational spectrum canvas uses; `tests/plots/capture.py` pictures them before and after a change](plots-share-the-spectrum-style.md)
 - [`ComputeMoCmd` caches a grid before computing it; a failed Compute must remove it (#239)](mo-grid-cache-keeps-a-failed-compute.md)
 - [The redraw sensor must not be drained inside `schedule()`: `SoWxEventHandler::setUpCallbacks()` only wakes idle](open-inventors-redraw-sensor-is-a-one.md)
 - [`ECCE_USE_COIN` (default ON) builds the viewer against Coin3D; `=OFF` keeps the vendored core for one release](ecce-use-coin-builds-the-viewer-against-coin3d.md)
@@ -31,6 +32,7 @@ Read this before changing any dialog, panel, sizer, grid, ewx control or the Ope
 - [`wxFIXED_MINSIZE`](wxfixed-minsize.md)
 - [Every drop-down is at least as wide as its widest entry](drop-down-min-width.md)
 - [Run-state colours: submitted yellow, created sky blue, pale fills outlined in light themes; checked by tests/look/contrast.py](run-state-colours.md)
+- [GTK :backdrop fades every label to ~2.8:1; `ewxApp::applyBackdropStyle()` undoes it; headless runs need `ECCE_TEST_BACKDROP` (#210)](gtk-backdrop-fades-labels-and-has-no-headless-trigger.md)
 - [`std::map`/`unordered_set` iterator invalidation](std-map-unordered-set-iterator-invalidation.md)
 - [Uncontrolled format strings](uncontrolled-format-strings.md)
 - [`EcceException::what()` returns storage that lives as long as the exception](ecceexception-what.md)
 
@@ -0,0 +1,40 @@
+---
+type: map
+title: "Every plot is an `ewxPlotCtrl` styled by `PlotStyle.H`, the palette the vibrational spectrum canvas uses; `tests/plots/capture.py` pictures them before and after a change"
+area: wx-viewer
+section: "wxWidgets 3.2/GTK3, the 3D viewer, and C++ pitfalls"
+paths: ["include/wxgui/PlotStyle.H", "include/wxgui/ewxPlotCtrl.H", "src/wxgui/ewxClasses/ewxPlotCtrl.C", "src/wxgui/wxplotctrl", "src/apps/builder/BuilderPlotShot.C", "tests/plots"]
+issues: [210, 214]
+---
+`SpectrumCanvas` (#214) is specific to spectra; what is shared is its
+colours. `PlotPalette` (was `SpectrumPalette`, still a typedef) lives in
+`include/wxgui/PlotStyle.H`, header-only so `tools/spectrum/render` needs
+no wx widgets. Every other plot is an `ewxPlotCtrl` (the old wxPlotCtrl
+under `src/wxgui/wxplotctrl`); `ewxPlotCtrl::ApplyStyle()` runs from
+`Create()` and gives it the system window colours, Okabe-Ito curves
+(`SeriesPen(i)`), small tick labels, short outside ticks, a pale grid, and
+a readout box for the point under the pointer. Drag zooms a rectangle,
+double click on empty plot fits everything, click still selects the point.
+
+- **A new plot panel needs nothing**; if it sets curve pens itself, use
+  `p_plotCtrl->SeriesPen(n)` and `GetPalette()`, not `wxBLUE`/`wxCYAN`.
+  A curve whose pens still equal the library default is restyled in
+  `StyleNewCurve()`; one with its own pens keeps them.
+- **`SetCorrectTicks(false)`.** The library's tick "correction" moved the
+  view to put ticks on round numbers and cut data off after a resize (the
+  Geometry Trace curve left the plot). Ticks now go where they fall.
+- **The y-axis window is as wide as its labels** (`CalcYAxisTickPositions`
+  re-lays out when the width changes); it used to be a fixed guess at
+  `-5.5e+555`. The window now ends at the plot frame so ticks reach it.
+- **`DrawAreaOverlay()` and `StyleNewCurve()`** are the two new virtual
+  hooks in `wxPlotCtrl`; the readout is drawn in the first.
+- **Rate-constant, equilibrium, MO-level and step plots all go through
+  this**; `PlotMeta3D`'s plot is commented out and draws nothing.
+- **Pictures.** `tests/plots/capture.py --png DIR --tag before|after`
+  floats each plot panel in the real Builder at two sizes (scene command
+  `plotshot`) and saves the pixels; `tests/plots/sheet.py DIR` makes the
+  contact sheets. One Builder per panel and size: resizing a floated
+  panel left stale pixels in the plot's native windows.
+- **`gtpick <step|mid|last>`** (scene script) clicks the plot where a step
+  is drawn and checks the molecule is the trace's; it is in
+  `tests/apps/geomtrace_stress.py`.
@@ -30,6 +30,10 @@ from the raw fixtures by `oracle.py` (no ECCE parser involved);
 - **The envelope is area-normalised**, so its y values are
   intensity per cm-1 and sit on a second, right-hand axis next to the
   stick axis; they are not comparable with the stick heights.
+- **The palette is shared.** `SpectrumPalette` is a typedef of
+  `PlotPalette` in `include/wxgui/PlotStyle.H`, which every other plot
+  uses too (see the plots entry); the spectrum paints byte-identically
+  with it.
 - **Hooks:** `ECCE_SPECTRUM_DUMP=<path>` (with `ECCE_SPECTRUM_CLICK=<mode>`
   and `ECCE_EXIT_AFTER_DUMP=1`) writes what the panel's canvas holds and
   paints it to `<path>.png`.
 
@@ -136,8 +136,13 @@ struct MoLevel
     */
    double         tabulated;
 
-   /** What the level is: see MoDiagram::classify(). */
-   enum Character { UNKNOWN, BONDING, NONBONDING, ANTIBONDING };
+   /**
+    * What the level is: see MoDiagram::classify().  MIXED is the middle
+    * orbital of a three-orbital interaction (water and ammonia 3a1):
+    * neither bonding nor antibonding by count, but with a real share on
+    * both sides, so it is drawn to both and not called non-bonding.
+    */
+   enum Character { UNKNOWN, BONDING, NONBONDING, ANTIBONDING, MIXED };
    Character      character;
 
    /**
@@ -390,12 +395,47 @@ class MoDiagram
      *
      * The tabulated energy is kept as the annotation, where it belongs.
      *
+     * Exception (#140): an atom's shell that has a non-bonding orbital
+     * in the molecular column (nonbondingShellFraction() at least
+     * NONBONDING_SHELL_SHARE) is drawn at that orbital's energy, so the
+     * orbital lies level with its parent.
+     *
      * @param connections  from connect(), which must therefore be
      *                     called first
+     * @return the number of shells drawn at a non-bonding orbital
      */
-    static void placeFragments(const MoColumn& centre,
-                               MoColumn& left, MoColumn& right,
-                               const vector<MoConnection>& connections);
+    static int placeFragments(const MoColumn& centre,
+                              MoColumn& left, MoColumn& right,
+                              const vector<MoConnection>& connections);
+
+    /**
+     * A molecular orbital whose nonbondingShellFraction() for one atom
+     * shell is at least this is that shell's non-bonding orbital (#140).
+     *
+     * Where symmetry forbids mixing (water's out-of-plane 2p) the
+     * fraction is 1 to the six decimals printed, in every basis tried.
+     * Allowed mixing, even when slight, left 0.055 or more elsewhere
+     * (extended Huckel NH3 and water 2a1: 0.055, 0.07; SCF water and
+     * NH3 3a1: 0.24, 0.14; nothing in CO or Cr(CO)6 above 0.88).  0.99
+     * keeps a factor of five from the nearest mixed orbital.
+     */
+    static constexpr double NONBONDING_SHELL_SHARE = 0.99;
+
+    /**
+     * How much of a molecular orbital is one atom shell, as a fraction
+     * of the part of it the diagram draws.
+     *
+     * The denominator is every shell of both columns plus any atom in
+     * neither column; shells of a column atom the diagram does not draw
+     * (polarisation functions: oxygen d, hydrogen p) are left out,
+     * since the diagram cannot show mixing with them.  -1 when the
+     * level carries no per-shell composition.
+     *
+     * @param onLeft  the shell is in the left column, else the right
+     * @param slot    index into that column's shellKeys
+     */
+    static double nonbondingShellFraction(const MoLevel& mo, bool onLeft,
+                                          int slot);
 
     /**
      * Fold levels below a cutoff into the column's hidden band.
@@ -706,9 +746,23 @@ class MoDiagram
                         const vector<MoLevel>& centre,
                         const vector<MoLevel>& right,
                         vector<MoConnection>& connections,
-                        double cutoff = 0.05,
+                        double cutoff = LINK_SHARE,
                         bool bothSidesQualify = false);
 
+    /**
+     * The least share of a molecular orbital, on the shells a column
+     * draws, for which connect() draws a line to that column; classify()
+     * uses the same number to decide that a level touches both sides.
+     */
+    static constexpr double LINK_SHARE = 0.05;
+
+    /**
+     * A level's share on the shells one column draws: the sum of its
+     * per-shell shares, or the column's whole share where there is no
+     * per-shell split, -1 where neither was computed.
+     */
+    static double drawnShare(const MoLevel& level, bool onLeft);
+
     /**
      * Fold levels ABOVE a cutoff, except ones a caller has protected.
      *
 
@@ -341,9 +341,19 @@ static bool buildModel(const char* tables, const char* molecule,
     MoDiagram::classify(left.levels, centre.levels, right.levels,
                         left.fromHalves);
     MoDiagram::connect(left.levels, centre.levels, right.levels, links);
-    MoDiagram::placeFragments(centre, left, right, links);
+    const int pinnedShells =
+        MoDiagram::placeFragments(centre, left, right, links);
     MoDiagram::classifyByEnergy(left.levels, centre.levels,
                                 right.levels, links);
+    //  Same sentence as MoDiagramPanel::build(), kept in step.
+    if (pinnedShells > 0) {
+      char text[200];
+      snprintf(text, sizeof(text), "A shell with a non-bonding orbital "
+               "(one that is at least %d%% that shell, of the orbitals "
+               "drawn) is drawn at that orbital's energy instead.",
+               (int)lround(100.0*MoDiagram::NONBONDING_SHELL_SHARE));
+      note = note.empty() ? string(text) : (note + " " + text);
+    }
   }