From e59bdb1485e6a82cc39fcf0dc7e0c4d94d89282a Mon Sep 17 00:00:00 2001
From: Raymond Hill <rhill@raymondhill.net>
Date: Mon, 6 May 2019 13:32:55 -0400
Subject: [PATCH] Defuse `fixed` position on `body` element in element zapper

The `fixed` style property on the `body` element will be
defused if an overlay element is removed using the element
zapper.

Related:
- https://www.reddit.com/r/uBlockOrigin/comments/bktxtb/scrolling_doesnt_work/emlscyz
---
 src/js/scriptlets/element-picker.js | 30 ++++++++++++++++++++++++-----
 1 file changed, 25 insertions(+), 5 deletions(-)

diff --git a/src/js/scriptlets/element-picker.js b/src/js/scriptlets/element-picker.js
index 2d04f9697..888bf4a77 100644
--- a/src/js/scriptlets/element-picker.js
+++ b/src/js/scriptlets/element-picker.js
@@ -1259,18 +1259,38 @@ const showDialog = function(options) {
 
 /******************************************************************************/
 
+// https://www.reddit.com/r/uBlockOrigin/comments/bktxtb/scrolling_doesnt_work/emn901o
+//   Override 'fixed' position property on body element if present.
+
 const zap = function() {
     if ( targetElements.length === 0 ) { return; }
+
+    const getStyleValue = function(elem, prop) {
+        const style = window.getComputedStyle(elem);
+        return style ? style[prop] : '';
+    };
+
     let elem = targetElements[0];
-    const style = window.getComputedStyle(elem);
     // Heuristic to detect scroll-locking: remove such lock when detected.
-    if ( parseInt(style.zIndex, 10) >= 1000 || style.position === 'fixed' ) {
-        document.body.style.setProperty('overflow', 'auto', 'important');
-        document.documentElement.style.setProperty('overflow', 'auto', 'important');
+    if (
+        parseInt(getStyleValue(elem, 'zIndex'), 10) >= 1000 ||
+        getStyleValue(elem, 'position') === 'fixed'
+    ) {
+        const doc = document;
+        if ( getStyleValue(doc.body, 'overflowY') === 'hidden' ) {
+            doc.body.style.setProperty('overflow', 'auto', 'important');
+        }
+        if ( getStyleValue(doc.body, 'position') === 'fixed' ) {
+            doc.body.style.setProperty('position', 'static', 'important');
+        }
+        if ( getStyleValue(doc.documentElement, 'overflowY') === 'hidden' ) {
+            doc.documentElement.style.setProperty('overflow', 'auto', 'important');
+        }
     }
+
     elem.parentNode.removeChild(elem);
     elem = elementFromPoint();
-    highlightElements(elem ? [elem] : []);
+    highlightElements(elem ? [ elem ] : []);
 };
 
 /******************************************************************************/