[FIX] web_editor: fix customize panel backdrop position

Before this commit, the customize panel backdrop did not fully cover the
customize panel when the vertical scrollbar was scrolled to the bottom.

Steps to reproduce the bug:

- In website edit mode, add a table of content snippet to the page.
- Add a three columns snippet within the table of content.
- Click on an image in the three columns snippet.
- Scroll the customize panel to the bottom and open the filter selector
of the image.
- Bug: the backdrop does not fully cover the customization panel.

task-3090626

closes odoo#114515

X-original-commit: 672a8cb
Signed-off-by: Outagant Mehdi (mou) <mou@odoo.com>

Author: bvr-odoo

addons/web_editor/static/src/scss/wysiwyg_snippets.scss

@@ -1720,8 +1720,14 @@
 
             &::after {
                 content: "";
-                @include o-position-absolute(0, 0, 0, 0);
+                // We use a "sticky" position because it ensures that the
+                // backdrop covers the entire "customize panel" element, even
+                // when it has a vertical scrollbar and is scrolled to the
+                // bottom.
+                @include o-position-sticky(auto, 0, 0, 0);
                 display: block;
+                height: 100vh;
+                margin-top: -100vh;
                 pointer-events: none;
                 background: $o-we-sidebar-content-backdrop-bg;
             }