From d690d4bed2cb1562fface956ed0c4e34e4e35c54 Mon Sep 17 00:00:00 2001
From: Pauli Virtanen <pav@iki.fi>
Date: Tue, 21 Apr 2020 01:19:39 +0300
Subject: [PATCH] Prevent PersistedElements overflowing scrolled areas

As the DOM element is not in reality contained inside "the parent",
it may overflow the area if the parent gets partially hidden by
scrolling etc.

To make the effect visually less annoying, emulate this by clipping to
the element wrapper. This is not a full general-purpose fix, but
improves the current situation.

Signed-off-by: Pauli Virtanen <pav@iki.fi>
---
 res/css/views/rooms/_AppsDrawer.scss          |  4 ++
 .../views/elements/PersistedElement.js        | 58 ++++++++++++++++++-
 2 files changed, 60 insertions(+), 2 deletions(-)

diff --git a/res/css/views/rooms/_AppsDrawer.scss b/res/css/views/rooms/_AppsDrawer.scss
index 1b1bab67bc..e4743f189e 100644
--- a/res/css/views/rooms/_AppsDrawer.scss
+++ b/res/css/views/rooms/_AppsDrawer.scss
@@ -96,6 +96,10 @@ $AppsDrawerBodyHeight: 273px;
     height: $AppsDrawerBodyHeight;
 }
 
+.mx_AppTile_persistedWrapper > div {
+    height: 100%;
+}
+
 .mx_AppTile_mini .mx_AppTile_persistedWrapper {
     height: 114px;
 }
diff --git a/src/components/views/elements/PersistedElement.js b/src/components/views/elements/PersistedElement.js
index 53f2501f19..18fa2aafef 100644
--- a/src/components/views/elements/PersistedElement.js
+++ b/src/components/views/elements/PersistedElement.js
@@ -156,16 +156,70 @@ export default class PersistedElement extends React.Component {
         child.style.display = visible ? 'block' : 'none';
     }
 
+    /*
+     * Clip element bounding rectangle to that of the parent elements.
+     * This is not a full visibility check, but prevents the persisted
+     * element from overflowing parent containers when inside a scrolled
+     * area.
+     */
+    _getClippedBoundingClientRect(element) {
+        let parentElement = element.parentElement;
+        let rect = element.getBoundingClientRect();
+
+        rect = new DOMRect(rect.left, rect.top, rect.width, rect.height);
+
+        while (parentElement) {
+            const parentRect = parentElement.getBoundingClientRect();
+
+            if (parentRect.left > rect.left) {
+                rect.width = rect.width - (parentRect.left - rect.left);
+                rect.x = parentRect.x;
+            }
+
+            if (parentRect.top > rect.top) {
+                rect.height = rect.height - (parentRect.top - rect.top);
+                rect.y = parentRect.y;
+            }
+
+            if (parentRect.right < rect.right) {
+                rect.width = rect.width - (rect.right - parentRect.right);
+            }
+
+            if (parentRect.bottom < rect.bottom) {
+                rect.height = rect.height - (rect.bottom - parentRect.bottom);
+            }
+
+            parentElement = parentElement.parentElement;
+        }
+
+        if (rect.width < 0) rect.width = 0;
+        if (rect.height < 0) rect.height = 0;
+
+        return rect;
+    }
+
     updateChildPosition(child, parent) {
         if (!child || !parent) return;
 
         const parentRect = parent.getBoundingClientRect();
+        const clipRect = this._getClippedBoundingClientRect(parent);
+
+        Object.assign(child.parentElement.style, {
+            position: 'absolute',
+            top: clipRect.top + 'px',
+            left: clipRect.left + 'px',
+            width: clipRect.width + 'px',
+            height: clipRect.height + 'px',
+            overflow: "hidden",
+        });
+
         Object.assign(child.style, {
             position: 'absolute',
-            top: parentRect.top + 'px',
-            left: parentRect.left + 'px',
+            top: (parentRect.top - clipRect.top) + 'px',
+            left: (parentRect.left - clipRect.left) + 'px',
             width: parentRect.width + 'px',
             height: parentRect.height + 'px',
+            overflow: "hidden",
         });
     }