Add center-visible-columns action

src/input/mod.rs

@@ -1445,6 +1445,11 @@ impl State {
                     self.niri.queue_redraw_all();
                 }
             }
+            Action::CenterVisibleColumns => {
+                self.niri.layout.center_visible_columns();
+                // FIXME: granular
+                self.niri.queue_redraw_all();
+            }
             Action::MaximizeColumn => {
                 self.niri.layout.toggle_full_width();
             }

src/layout/mod.rs

@@ -2332,6 +2332,13 @@ impl<W: LayoutElement> Layout<W> {
         workspace.center_window(id);
     }
 
+    pub fn center_visible_columns(&mut self) {
+        let Some(workspace) = self.active_workspace_mut() else {
+            return;
+        };
+        workspace.center_visible_columns();
+    }
+
     pub fn focus(&self) -> Option<&W> {
         self.focus_with_output().map(|(win, _out)| win)
     }

src/layout/scrolling.rs

@@ -2139,6 +2139,64 @@ impl<W: LayoutElement> ScrollingSpace<W> {
         self.center_column();
     }
 
+    pub fn center_visible_columns(&mut self) {
+        if self.columns.is_empty() {
+            return;
+        }
+
+        if self.is_centering_focused_column() {
+            return;
+        }
+
+        // Consider the end of an ongoing animation because that's what compute to fit does too.
+        let view_x = self.target_view_pos();
+        let working_x = self.working_area.loc.x;
+        let working_w = self.working_area.size.w;
+
+        // Count all columns that are fully visible inside the working area.
+        let mut width_taken = 0.;
+        let mut leftmost_col_x = None;
+        let mut active_col_x = None;
+
+        let gap = self.options.gaps;
+        let col_xs = self.column_xs(self.data.iter().copied());
+        for (idx, col_x) in col_xs.take(self.columns.len()).enumerate() {
+            if col_x < view_x + working_x + gap {
+                // Column goes off-screen to the left.
+                continue;
+            }
+
+            leftmost_col_x.get_or_insert(col_x);
+
+            let width = self.data[idx].width;
+            if view_x + working_x + working_w < col_x + width + gap {
+                // Column goes off-screen to the right. We can stop here.
+                break;
+            }
+
+            if idx == self.active_column_idx {
+                active_col_x = Some(col_x);
+            }
+
+            width_taken += width + gap;
+        }
+
+        if active_col_x.is_none() {
+            // The active column wasn't fully on screen, so we can't meaningfully do anything.
+            return;
+        }
+
+        let col = &mut self.columns[self.active_column_idx];
+        cancel_resize_for_column(&mut self.interactive_resize, col);
+
+        let free_space = working_w - width_taken + gap;
+        let new_view_x = leftmost_col_x.unwrap() - free_space / 2. - working_x;
+
+        self.animate_view_offset(self.active_column_idx, new_view_x - active_col_x.unwrap());
+        // Just in case.
+        self.animate_view_offset_to_column(None, self.active_column_idx, None);
+    }
+
     pub fn view_pos(&self) -> f64 {
         self.column_x(self.active_column_idx) + self.view_offset.current()
     }

src/layout/tests.rs

@@ -464,6 +464,7 @@ enum Op {
         #[proptest(strategy = "proptest::option::of(1..=5usize)")]
         id: Option<usize>,
     },
+    CenterVisibleColumns,
     FocusWorkspaceDown,
     FocusWorkspaceUp,
     FocusWorkspace(#[proptest(strategy = "0..=4usize")] usize),
@@ -1068,6 +1069,7 @@ impl Op {
                 let id = id.filter(|id| layout.has_window(id));
                 layout.center_window(id.as_ref());
             }
+            Op::CenterVisibleColumns => layout.center_visible_columns(),
             Op::FocusWorkspaceDown => layout.switch_workspace_down(),
             Op::FocusWorkspaceUp => layout.switch_workspace_up(),
             Op::FocusWorkspace(idx) => layout.switch_workspace(idx),

src/layout/workspace.rs

@@ -1100,6 +1100,13 @@ impl<W: LayoutElement> Workspace<W> {
         }
     }
 
+    pub fn center_visible_columns(&mut self) {
+        if self.floating_is_active.get() {
+            return;
+        }
+        self.scrolling.center_visible_columns();
+    }
+
     pub fn toggle_width(&mut self) {
         if self.floating_is_active.get() {
             self.floating.toggle_window_width(None);