diff --git a/app/assets/stylesheets/pages/detail_page.scss b/app/assets/stylesheets/pages/detail_page.scss
index 7b36bcb3c7d..2e007c52592 100644
--- a/app/assets/stylesheets/pages/detail_page.scss
+++ b/app/assets/stylesheets/pages/detail_page.scss
@@ -23,7 +23,8 @@
   position: relative;
   line-height: 35px;
   display: flex;
-  flex-grow: 1;
+  flex: 1 1;
+  min-width: 0;
 
   @include media-breakpoint-up(sm) {
     padding-left: 0;
diff --git a/app/assets/stylesheets/pages/issuable.scss b/app/assets/stylesheets/pages/issuable.scss
index b42c232fd91..f9fd9f1ab8b 100644
--- a/app/assets/stylesheets/pages/issuable.scss
+++ b/app/assets/stylesheets/pages/issuable.scss
@@ -698,6 +698,8 @@
   font-size: 14px;
   line-height: 24px;
   align-self: center;
+  overflow: hidden;
+  text-overflow: ellipsis;
 }
 
 .js-issuable-selector-wrap {
diff --git a/app/helpers/issuables_helper.rb b/app/helpers/issuables_helper.rb
index d42284868c7..9f501ea55fb 100644
--- a/app/helpers/issuables_helper.rb
+++ b/app/helpers/issuables_helper.rb
@@ -157,7 +157,7 @@ module IssuablesHelper
     output = ""
     output << "Opened #{time_ago_with_tooltip(issuable.created_at)} by ".html_safe
     output << content_tag(:strong) do
-      author_output = link_to_member(project, issuable.author, size: 24, mobile_classes: "d-none d-sm-inline-block", tooltip: true)
+      author_output = link_to_member(project, issuable.author, size: 24, mobile_classes: "d-none d-sm-inline", tooltip: true)
       author_output << link_to_member(project, issuable.author, size: 24, by_username: true, avatar: false, mobile_classes: "d-block d-sm-none")
     end
 
diff --git a/changelogs/unreleased/46861-issuable-title-with-longer-username.yml b/changelogs/unreleased/46861-issuable-title-with-longer-username.yml
new file mode 100644
index 00000000000..9df6879deb6
--- /dev/null
+++ b/changelogs/unreleased/46861-issuable-title-with-longer-username.yml
@@ -0,0 +1,5 @@
+---
+title: Fix CSS for buttons not to be hidden on issues/MR title
+merge_request: 19176
+author: Takuya Noguchi
+type: fixed