Add a previous event safe area around action bar

This adds a previous event safe area so that you can't accidentally hover the
previous event while trying to mouse into the action bar or from the react
button to its tooltip.

Part of element-hq/element-web#10185
Fixes element-hq/element-web#9650

res/css/views/messages/_MessageActionBar.scss

@@ -1,5 +1,6 @@
 /*
 Copyright 2019 New Vector Ltd
+Copyright 2019 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -29,6 +30,22 @@ limitations under the License.
     // Ensure the action bar appears above over things, like the read marker.
     z-index: 1;
 
+    // Adds a previous event safe area so that you can't accidentally hover the
+    // previous event while trying to mouse into the action bar or from the
+    // react button to its tooltip.
+    &::before {
+        content: '';
+        position: absolute;
+        // tooltip overhang + action bar + action bar offset from event
+        width: calc(48px + 100% + 8px);
+        // safe area + action bar
+        height: calc(20px + 100%);
+        top: -20px;
+        left: -48px;
+        z-index: -1;
+        cursor: initial;
+    }
+
     > * {
         white-space: nowrap;
         display: inline-block;