[ruler] css patch for notebook > 4.2.3

see jupyter/notebook#2869 for details

src/jupyter_contrib_nbextensions/nbextensions/ruler/main.js

@@ -13,6 +13,7 @@ define([
     var ruler_column = [78];
     var ruler_color = ["#ff0000"];
     var ruler_linestyle = ["dashed"];
+    var ruler_do_css_patch;
 
     var rulers = [];
 
@@ -51,6 +52,8 @@ define([
             });
         }
         console.debug(log_prefix, 'ruler specs:', rulers);
+
+        ruler_do_css_patch = config.data.ruler_do_css_patch; // undefined is ok
     }
 
     var load_ipython_extension = function() {
@@ -59,13 +62,29 @@ define([
             console.warn(log_prefix, 'error loading config:', reason);
         })
         .then(function () {
+            // Add css patch fix for notebook > 4.2.3 - see
+            //     https://github.com/jupyter/notebook/issues/2869
+            // for details
+            if (ruler_do_css_patch !== false) {
+                var sheet = document.createElement('style');
+                sheet.innerHTML = [
+                    '.CodeMirror-lines {',
+                    '  padding: 0.4em 0; /* Vertical padding around content */',
+                    '}',
+                    '.CodeMirror pre {',
+                    '  padding: 0 0.4em; /* Horizonal padding around content */',
+                    '}',
+                ].join('\n');
+                document.head.appendChild(sheet);
+            }
+
             // Change default for new cells
             codecell.CodeCell.options_default.cm_config.rulers = rulers;
             // Apply to any already-existing cells
             var cells = Jupyter.notebook.get_cells().forEach(function (cell) {
                 if (cell instanceof codecell.CodeCell) {
                     cell.code_mirror.setOption('rulers', rulers);
-                } 
+                }
             });
         })
         .catch(function on_error (reason) {

src/jupyter_contrib_nbextensions/nbextensions/ruler/readme.md

@@ -1,5 +1,6 @@
 Ruler
 =====
+
 This extension enables the Ruler CodeMirror feature
 
 
@@ -15,8 +16,21 @@ cm = ConfigManager(parent=ip)
 cm.update('notebook', {"ruler_column": [80]})
 ```
 
+
+#### CSS patch ####
+
+Notebook versions from 4.3.0 through 5.1.0dev show up a bug in their CodeMirror
+CSS padding which causes the ruler to be misplaced (see
+[jupyter/notebook#2869](https://github.com/jupyter/notebook/issues/2869)
+for details).
+This nbextension introduces a css patch to attempt to correct this, but if it
+causes problems for you, you can disable it by setting the `ruler_do_css_patch`
+config key to `false`.
+
+
 #### Multiple Rulers ####
-To specify multiple rulers, set the `ruler_column` to an list of values, for example
+
+To specify multiple rulers, set the `ruler_column` to a list of values, for example
 
 ```Python
 cm.update('notebook', {"ruler_column": [10, 20, 30, 40, 50, 60, 70, 80]})

src/jupyter_contrib_nbextensions/nbextensions/ruler/ruler.yaml

@@ -6,19 +6,27 @@ Icon: icon.png
 Main: main.js
 Compatibility: 4.x, 5.x
 Parameters:
+
 - name: ruler_column
   input_type: list
   list_element:
     input_type: number
   description: Column where ruler is displayed
   default: [78]
+
 - name: ruler_color
   input_type: list
   list_element:
     input_type: color
   description: Ruler color
   default: ["#ff0000"]
+
 - name: ruler_linestyle
   description: 'Ruler style, e.g. solid, dashed'
   input_type: list
   default: ['dashed']
+
+- name: ruler_do_css_patch
+  description: apply css patch for ruler padding bug in notebook >= 4.3
+  input_type: checkbox
+  default: true