fix: footer margin handling (Closes #6616 )

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## Unreleased
+
+### Fixed
+
+- Fixed Footer's width calculation when docked to correctly respect margins https://github.com/Textualize/textual/issues/6616
+
 ## [8.2.8] - 2026-06-30
 
 ### Fixed

src/textual/widgets/_footer.py

@@ -154,6 +154,7 @@ class Footer(ScrollableContainer, can_focus=False, can_focus_children=False):
         background: $footer-background;
         dock: bottom;
         height: 1;
+        width: 100%;
         scrollbar-size: 0 0;
         &.-compact {
             FooterLabel {

tests/snapshot_tests/snapshot_apps/footer_margin.py

@@ -0,0 +1,29 @@
+from textual.app import App, ComposeResult
+from textual.widgets import Footer
+
+
+class FooterMarginApp(App):
+    """Regression test for https://github.com/Textualize/textual/issues/6616
+
+    Setting a margin on the Footer should shrink it on all sides. Previously
+    the Footer kept a fixed width, so the left/top margin created a gap but
+    the right margin had nowhere to go and was invisible."""
+
+    CSS = """
+    Screen {
+        background: blue;
+    }
+    """
+
+    BINDINGS = [("q", "quit", "Quit")]
+
+    def compose(self) -> ComposeResult:
+        yield Footer()
+
+    def on_mount(self) -> None:
+        self.query_one(Footer).styles.margin = 5
+
+
+if __name__ == "__main__":
+    app = FooterMarginApp()
+    app.run()

tests/snapshot_tests/test_snapshots.py

@@ -391,7 +391,11 @@ def test_datatable_change_cell_padding(snap_compare):
 def test_footer_render(snap_compare):
     assert snap_compare(WIDGET_EXAMPLES_DIR / "footer.py")
 
+def test_footer_margin(snap_compare):
+    """Regression test for https://github.com/Textualize/textual/issues/6616"""
+    assert snap_compare(SNAPSHOT_APPS_DIR / "footer_margin.py")
 
+
 def test_header_render(snap_compare):
     assert snap_compare(WIDGET_EXAMPLES_DIR / "header.py")