(css) Constrain absolute elements in HTML parts

NEWS

@@ -15,6 +15,7 @@ Bug fixes
  - [web] check for duplicate name only if address book name is changed
  - [web] improved detection of URLs and email addresses in text mail parts
  - [web] fixed page reload with external IMAP account (#4709)
+ - [web] constrained absolute-positioned child elements of HTML mail parts
  - [core] allow super users to modify any event (#4216)
  - [core] correctly handle the full cert chain in S/MIME
  - [core] handle multidays events in freebusy data

UI/WebServerResources/scss/views/MailerUI.scss

@@ -252,11 +252,16 @@ md-sidenav {
   }
 }
 
-// Force wrapping of pre elements in HTML content
-.mailer_htmlcontent pre {
-  white-space: pre-wrap;
-  word-wrap: break-word;
-  tab-size: 4;
+.mailer_htmlcontent {
+  // Constrain absolute-positioned child elements to this element
+  position: relative;
+
+  // Force wrapping of pre elements in HTML content
+  pre {
+    white-space: pre-wrap;
+    word-wrap: break-word;
+    tab-size: 4;
+  }
 }
 
 .sg-mail-editor {