diff --git a/src/__celerity_resource_map__.php b/src/__celerity_resource_map__.php
index 06febdb093..93cf2d638f 100644
--- a/src/__celerity_resource_map__.php
+++ b/src/__celerity_resource_map__.php
@@ -839,6 +839,17 @@ celerity_register_resource_map(array(
     ),
     'disk' => '/rsrc/js/application/core/behavior-object-selector.js',
   ),
+  'javelin-behavior-phabricator-oncopy' =>
+  array(
+    'uri' => '/res/70b9b75e/rsrc/js/application/core/behavior-oncopy.js',
+    'type' => 'js',
+    'requires' =>
+    array(
+      0 => 'javelin-behavior',
+      1 => 'javelin-dom',
+    ),
+    'disk' => '/rsrc/js/application/core/behavior-oncopy.js',
+  ),
   'javelin-behavior-phabricator-tooltips' =>
   array(
     'uri' => '/res/49f92a92/rsrc/js/application/core/behavior-tooltip.js',
diff --git a/src/applications/differential/parser/changeset/DifferentialChangesetParser.php b/src/applications/differential/parser/changeset/DifferentialChangesetParser.php
index 78b3980a59..0440d28494 100644
--- a/src/applications/differential/parser/changeset/DifferentialChangesetParser.php
+++ b/src/applications/differential/parser/changeset/DifferentialChangesetParser.php
@@ -1417,7 +1417,10 @@ final class DifferentialChangesetParser {
           '<th'.$o_id.'>'.$o_num.'</th>'.
           '<td'.$o_attr.'>'.$o_text.'</td>'.
           '<th'.$n_id.'>'.$n_num.'</th>'.
-          '<td'.$n_attr.'>'.$n_text.'</td>'.
+          // NOTE: This is a unicode zero-width space, which we use as a hint
+          // when intercepting 'copy' events to make sure sensible text ends
+          // up on the clipboard. See the 'phabricator-oncopy' behavior.
+          '<td'.$n_attr.'>'."\xE2\x80\x8B".$n_text.'</td>'.
           $n_cov.
         '</tr>';
 
diff --git a/src/applications/differential/view/changesetdetailview/DifferentialChangesetDetailView.php b/src/applications/differential/view/changesetdetailview/DifferentialChangesetDetailView.php
index c6134b09ee..f3d9b6629a 100644
--- a/src/applications/differential/view/changesetdetailview/DifferentialChangesetDetailView.php
+++ b/src/applications/differential/view/changesetdetailview/DifferentialChangesetDetailView.php
@@ -65,6 +65,8 @@ final class DifferentialChangesetDetailView extends AphrontView {
     require_celerity_resource('differential-changeset-view-css');
     require_celerity_resource('syntax-highlighting-css');
 
+    Javelin::initBehavior('phabricator-oncopy', array());
+
     if ($this->revisionID) {
       $edit = true;
     } else {
diff --git a/src/applications/paste/controller/view/PhabricatorPasteViewController.php b/src/applications/paste/controller/view/PhabricatorPasteViewController.php
index 48ecbea65f..26ccb1d7b9 100644
--- a/src/applications/paste/controller/view/PhabricatorPasteViewController.php
+++ b/src/applications/paste/controller/view/PhabricatorPasteViewController.php
@@ -142,6 +142,7 @@ final class PhabricatorPasteViewController extends PhabricatorPasteController {
 
     $text_list = explode("\n", $source);
 
+    Javelin::initBehavior('phabricator-oncopy', array());
     $rows = $this->buildDisplayRows($text_list);
 
     $corpus_table = phutil_render_tag(
@@ -179,7 +180,10 @@ final class PhabricatorPasteViewController extends PhabricatorPasteController {
         ),
         $n);
       $rows[] = '<tr id="'.$anchor.'"><th>'.$link.'</th>'.
-        '<td style="white-space: pre-wrap;">'.$line.'</td></tr>';
+        '<td style="white-space: pre-wrap;">'.
+        // NOTE: See the 'phabricator-oncopy' behavior.
+        "\xE2\x80\x8B".
+        $line.'</td></tr>';
       ++$n;
     }
 
diff --git a/src/applications/paste/controller/view/__init__.php b/src/applications/paste/controller/view/__init__.php
index 892964bc8c..57ad86603b 100644
--- a/src/applications/paste/controller/view/__init__.php
+++ b/src/applications/paste/controller/view/__init__.php
@@ -13,6 +13,7 @@ phutil_require_module('phabricator', 'applications/markup/syntax');
 phutil_require_module('phabricator', 'applications/paste/controller/base');
 phutil_require_module('phabricator', 'applications/paste/storage/paste');
 phutil_require_module('phabricator', 'infrastructure/celerity/api');
+phutil_require_module('phabricator', 'infrastructure/javelin/api');
 phutil_require_module('phabricator', 'view/control/table');
 phutil_require_module('phabricator', 'view/layout/panel');
 
diff --git a/webroot/rsrc/js/application/core/behavior-oncopy.js b/webroot/rsrc/js/application/core/behavior-oncopy.js
new file mode 100644
index 0000000000..fb3b6bfd06
--- /dev/null
+++ b/webroot/rsrc/js/application/core/behavior-oncopy.js
@@ -0,0 +1,80 @@
+/**
+ * @provides javelin-behavior-phabricator-oncopy
+ * @requires javelin-behavior
+ *           javelin-dom
+ */
+
+/**
+ * Tools like Paste and Differential don't normally respond to the clipboard
+ * 'copy' operation well, because when a user copies text they'll get line
+ * numbers and other metadata.
+ *
+ * To improve this behavior, applications can embed markers that delimit
+ * metadata (left of the marker) from content (right of the marker). When
+ * we get a copy event, we strip out all the metadata and just copy the
+ * actual text.
+ */
+JX.behavior('phabricator-oncopy', function() {
+
+  var zws = "\u200B"; // Unicode Zero-Width Space
+
+  document.body.oncopy = function(e) {
+
+    var selection = window.getSelection();
+    var text = selection.toString();
+
+    if (text.indexOf(zws) == -1) {
+      // If there's no marker in the text, just let it copy normally.
+      return;
+    }
+
+    var result = [];
+
+    // Strip everything before the marker (and the marker itself) out of the
+    // text. If a line doesn't have the marker, throw it away (the assumption
+    // is that it's a line number or part of some other meta-text).
+    var lines = text.split("\n");
+    var pos;
+    for (var ii = 0; ii < lines.length; ii++) {
+      pos = lines[ii].indexOf(zws);
+      if (pos == -1 && ii != 0) {
+        continue;
+      }
+      result.push(lines[ii].substring(pos + 1));
+    }
+    result = result.join("\n");
+
+    if (e.clipboardData) {
+      // Safari and Chrome support this easy, straightforward mechanism.
+      e.clipboardData.setData('Text', result);
+      e.preventDefault();
+    } else {
+
+      // In Firefox, we have to create a <pre> and select the text in it, then
+      // let the copy event fire. It has to be a <pre> because Firefox won't
+      // copy returns properly out of a div, even if it has 'whitespace: pre'.
+      // There's been a bug open for 10 (!) years:
+      //
+      //   https://bugzilla.mozilla.org/show_bug.cgi?id=116083
+
+      var style = {
+        position: 'absolute',
+        left:     '-10000px'
+      };
+      var pre = JX.$N('pre', style, result);
+      document.body.appendChild(pre);
+
+      // Select the text in the <pre>.
+      var range = document.createRange();
+      range.selectNodeContents(pre);
+      selection.removeAllRanges();
+      selection.addRange(range);
+
+      setTimeout(function() { JX.DOM.remove(pre) }, 0);
+
+      // TODO: I tried to restore the old selection range but it doesn't seem
+      // to work or give me any errors. So you lose your selection when you
+      // copy. Oh well?
+    }
+  }
+});