diff --git a/addons/mail/static/src/components/chatter/chatter.scss b/addons/mail/static/src/components/chatter/chatter.scss
index ee4c00855b4..d722e03b93b 100644
--- a/addons/mail/static/src/components/chatter/chatter.scss
+++ b/addons/mail/static/src/components/chatter/chatter.scss
@@ -7,6 +7,7 @@
     display: flex;
     flex: 1 1 auto;
     flex-direction: column;
+    width: map-get($sizes, 100);
 }
 
 .o_Chatter_composer {
diff --git a/addons/mail/static/src/components/chatter_container/chatter_container.scss b/addons/mail/static/src/components/chatter_container/chatter_container.scss
index 623f902eaf7..8cd51580ec9 100644
--- a/addons/mail/static/src/components/chatter_container/chatter_container.scss
+++ b/addons/mail/static/src/components/chatter_container/chatter_container.scss
@@ -5,6 +5,7 @@
 .o_ChatterContainer {
     display: flex;
     flex: 1 1 auto;
+    width: map-get($sizes, 100);
 }
 
 .o_ChatterContainer_noChatter {