diff --git a/.changeset/easy-cobras-design.md b/.changeset/easy-cobras-design.md
new file mode 100644
index 0000000000..693eb3bd4d
--- /dev/null
+++ b/.changeset/easy-cobras-design.md
@@ -0,0 +1,5 @@
+---
+'@openproject/primer-view-components': patch
+---
+
+Fix `CollapsibleHeader` title truncation when `multi_line: false`.
diff --git a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/alpha/text_area/with_character_limit_over_limit/dark_colorblind.png b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/alpha/text_area/with_character_limit_over_limit/dark_colorblind.png
index bec83a07fd..a9f1f2f611 100644
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diff --git a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/alpha/text_area/with_character_limit_over_limit/light.png b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/alpha/text_area/with_character_limit_over_limit/light.png
index a405f0b63c..6dda568924 100644
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diff --git a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/alpha/text_area/with_character_limit_over_limit/light_colorblind.png b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/alpha/text_area/with_character_limit_over_limit/light_colorblind.png
index 8b7c4900e5..323cd3d2d9 100644
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diff --git a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/alpha/text_area/with_character_limit_over_limit/light_high_contrast.png b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/alpha/text_area/with_character_limit_over_limit/light_high_contrast.png
index a6b224f774..d6d8d5a7ea 100644
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diff --git a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/alpha/text_field/with_character_limit_over_limit/dark_dimmed.png b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/alpha/text_field/with_character_limit_over_limit/dark_dimmed.png
index 84b8475ca1..b55755348c 100644
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diff --git a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/alpha/text_field/with_character_limit_over_limit/light.png b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/alpha/text_field/with_character_limit_over_limit/light.png
index ad56d0d4ed..a890f61506 100644
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diff --git a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/alpha/text_field/with_character_limit_over_limit/light_high_contrast.png b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/alpha/text_field/with_character_limit_over_limit/light_high_contrast.png
index 017774d6f4..2756ff889f 100644
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diff --git a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/collapsed/default.png b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/collapsed/default.png
index 8a65440975..b7e47665e7 100644
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diff --git a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/collapsed/focused.png b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/collapsed/focused.png
index 26abc5637d..58811e0227 100644
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diff --git a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/default/default.png b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/default/default.png
index 9b433c1af6..841143623b 100644
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diff --git a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/default/focused.png b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/default/focused.png
index 2763dead4c..b762a0a770 100644
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diff --git a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/with_count/default.png b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/with_count/default.png
index 96a8803ad2..ad8d469a26 100644
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diff --git a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/with_count/focused.png b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/with_count/focused.png
index de1cc4d91f..711c5625c0 100644
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diff --git a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/with_description/default.png b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/with_description/default.png
index fd0392de36..df6e578bbc 100644
Binary files a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/with_description/default.png and b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/with_description/default.png differ
diff --git a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/with_description/focused.png b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/with_description/focused.png
index 3aa37503c6..c3e71bb950 100644
Binary files a/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/with_description/focused.png and b/.playwright/screenshots/snapshots.test.ts-snapshots/primer/open_project/border_box/collapsible_header/with_description/focused.png differ
diff --git a/app/components/primer/open_project/border_box/collapsible_header.pcss b/app/components/primer/open_project/border_box/collapsible_header.pcss
index 561e6ac7f3..2be7fd3a3a 100644
--- a/app/components/primer/open_project/border_box/collapsible_header.pcss
+++ b/app/components/primer/open_project/border_box/collapsible_header.pcss
@@ -26,6 +26,7 @@
   align-items: center;
   column-gap: var(--stack-gap-normal);
   row-gap: var(--base-size-4);
+  overflow: hidden;
 
   .CollapsibleHeader > & {
     flex: 1;
@@ -66,10 +67,12 @@
 }
 
 .CollapsibleHeader-title-line {
+  flex-basis: 66%;
   display: flex;
   align-items: center;
   gap: var(--stack-gap-condensed);
-  min-width: fit-content;
+  overflow: hidden;
+  max-width: 100%;
 }
 
 .CollapsibleHeader-description {
diff --git a/app/components/primer/open_project/border_box/collapsible_header.rb b/app/components/primer/open_project/border_box/collapsible_header.rb
index 7ce74202ae..c4d0a99551 100644
--- a/app/components/primer/open_project/border_box/collapsible_header.rb
+++ b/app/components/primer/open_project/border_box/collapsible_header.rb
@@ -22,7 +22,7 @@ class CollapsibleHeader < Primer::Component
             "Box-title"
           )
 
-          Primer::Beta::Heading.new(tag: fetch_or_fallback(TITLE_TAG_OPTIONS, tag, TITLE_TAG_DEFAULT), **system_arguments)
+          Primer::Beta::Truncate.new(tag: fetch_or_fallback(TITLE_TAG_OPTIONS, tag, TITLE_TAG_DEFAULT), **system_arguments)
         }
 
         # Optional count