Add focus button.

README.md

@@ -283,6 +283,9 @@ in principle, but a terminal grid is small.
   [Agent status](#agent-status)
 - **Renaming a tab**: `Ctrl+Shift+R`, right-click or double-click a tab row.
   A typed name pins the label; clearing it hands the label back to the panes
+- **Zooming a pane** to fill its tab and back, from the header button or
+  `Ctrl+Shift+Z`. Nothing closes and nothing moves — hidden panes keep running
+  and the split comes back exactly as it was. See [Zoom](#zoom)
 
 ### Shortcuts
 
@@ -295,6 +298,7 @@ in principle, but a terminal grid is small.
 | `Ctrl+Shift+←/→/↑/↓` | move the focused pane within its view |
 | `Ctrl+Shift+V` | paste into a terminal (bracketed-paste aware, refuses unsafe pastes) |
 | `Ctrl+Shift+R` | rename the current tab (empty name = follow the terminal) |
+| `Ctrl+Shift+Z` | zoom the focused pane to fill the tab, and back |
 | `Ctrl+PageUp/PageDown` | previous / next tab |
 | `Alt+1`..`Alt+8` | jump to tab N, `Alt+9` jumps to the last |
 
@@ -306,6 +310,31 @@ competes with the content's own mouse handling.
 Both add-pane shortcuts are also buttons in every pane header, which is how you
 open a web view without remembering the chord.
 
+## Zoom
+
+A split that was the right shape for watching two things at once is usually the
+wrong shape for actually working in one of them. `Ctrl+Shift+Z`, or the
+fullscreen button in a pane's header, gives that pane the whole tab; the same
+again puts the split back.
+
+This is not the window manager's fullscreen. The window keeps its decorations,
+the sidebar stays put, and only the tab's own content area is involved.
+
+Nothing closes and nothing is rearranged. The split tree is left exactly as it
+was and only the rendering changes, which is what lets the restore be exact
+rather than an approximation — ratios, orientations and ordering all come back
+untouched, because they were never taken apart. Hidden panes are unparented but
+still alive: their shells keep running, output produced while they were off
+screen is waiting when they return, and their grids keep the size they had
+rather than reflowing to nothing.
+
+The toggle only appears once a tab holds more than one pane, since zooming the
+only pane in a view would be an invisible state change. Splitting a pane or
+moving one leaves zoom, because both of those actions are about the arrangement
+that zoom is hiding. Closing the zoomed pane leaves zoom; a *different* pane
+closing — a background shell exiting, say — does not, since what you are looking
+at is still there.
+
 ## Agent status
 
 Playpen is mostly used to keep several Claude Code sessions side by side, and

src/Layout.zig

@@ -269,6 +269,27 @@ pub fn materialize(self: *Layout, box: *gtk.Box) void {
     box.append(root.widget());
 }
 
+/// Mount a single pane filling `box`, hiding the rest of the tree.
+///
+/// The tree itself is left completely alone — no node is moved, removed or
+/// reparented in the model — so this is purely a change of what is on screen
+/// and `materialize` puts the original arrangement back exactly. That is the
+/// whole reason zoom is done here rather than by restructuring: an arrangement
+/// that had to be taken apart and rebuilt would have to preserve split ratios,
+/// orientations and ordering by hand, and would drift.
+///
+/// Detaching the whole tree first is what frees the zoomed pane from the paned
+/// holding it. The other panes end up parentless for the duration, which is
+/// harmless: every pane holds a strong reference to its own widget, and a
+/// terminal's PTY is watched on the main loop rather than driven by its
+/// widget, so a hidden pane's shell keeps running and simply redraws from the
+/// terminal state it accumulated when it comes back.
+pub fn materializeZoom(self: *Layout, box: *gtk.Box, pane: *Pane) void {
+    if (box.as(gtk.Widget).getFirstChild()) |child| box.remove(child);
+    if (self.root) |root| detach(root);
+    box.append(pane.widget());
+}
+
 fn detach(node: *Node) void {
     switch (node.kind) {
         .leaf => {},

src/Pane.zig

@@ -127,6 +127,11 @@ const all_sides = [_]Side{ .left, .right, .top, .bottom };
 /// The dot itself. A filled circle at whatever size the context asks for.
 pub const status_icon = "media-record-symbolic";
 
+/// Zoom toggle icons. Deliberately not the system-fullscreen pair's meaning:
+/// this fills the tab, not the screen, and the window keeps its decorations.
+const zoom_icon = "view-fullscreen-symbolic";
+const restore_icon = "view-restore-symbolic";
+
 /// Every class a dot can carry. Listed so that applying one can clear the
 /// others without the caller having to remember what it set last.
 pub const status_classes = [_][:0]const u8{
@@ -182,6 +187,10 @@ box: *gtk.Box,
 header: *gtk.Box,
 label: *gtk.Label,
 
+/// Zoom toggle, kept so its icon can follow the view's zoom state and so it
+/// can be hidden in a view too small for zoom to mean anything.
+zoom: *gtk.Button,
+
 /// Status dot in the header, hidden while the pane is idle. This is the
 /// per-pane counterpart of the dot on the sidebar row: with several panes in
 /// a tab, the row tells you the tab needs attention and this tells you which
@@ -209,6 +218,7 @@ pub fn create(alloc: std.mem.Allocator, view: *View, spec: Spec) !*Pane {
         .header = gtk.Box.new(.horizontal, 4),
         .label = gtk.Label.new(""),
         .dot = gtk.Image.newFromIconName(status_icon),
+        .zoom = gtk.Button.newFromIconName(zoom_icon),
     };
     setTitle(self, kind.initialTitle());
 
@@ -328,6 +338,20 @@ fn buildHeader(self: *Pane) void {
     _ = gtk.Button.signals.clicked.connect(web, *Pane, &onWebClicked, self, .{});
     header.append(web.as(gtk.Widget));
 
+    // Last before close, so the destructive button stays on the end where it
+    // is expected and the zoom toggle sits with the other view controls.
+    self.zoom.as(gtk.Widget).addCssClass("flat");
+    self.zoom.as(gtk.Widget).addCssClass("playpen-pane-button");
+    _ = gtk.Button.signals.clicked.connect(self.zoom, *Pane, &onZoomClicked, self, .{});
+    header.append(self.zoom.as(gtk.Widget));
+
+    // Starts hidden. A pane is created as the only one in its view often
+    // enough — a new tab, the first pane of a layout — that showing a control
+    // which cannot do anything yet would be the common case, not the corner
+    // one. The view turns it on as soon as there is a second pane.
+    self.setZoomed(false);
+    self.setZoomAvailable(false);
+
     const close = gtk.Button.newFromIconName("window-close-symbolic");
     close.as(gtk.Widget).addCssClass("flat");
     close.as(gtk.Widget).addCssClass("playpen-pane-button");
@@ -336,6 +360,27 @@ fn buildHeader(self: *Pane) void {
     header.append(close.as(gtk.Widget));
 }
 
+/// Show this pane as the zoomed one, or as an ordinary pane again.
+pub fn setZoomed(self: *Pane, on: bool) void {
+    self.zoom.setIconName(if (on) restore_icon else zoom_icon);
+    self.zoom.as(gtk.Widget).setTooltipText(if (on)
+        "Restore the other panes (Ctrl+Shift+Z)"
+    else
+        "Fill the tab with this pane (Ctrl+Shift+Z)");
+
+    if (on) {
+        self.widget().addCssClass("zoomed");
+    } else {
+        self.widget().removeCssClass("zoomed");
+    }
+}
+
+/// Hide the toggle in a view holding only this pane, where it would have
+/// nothing to do.
+pub fn setZoomAvailable(self: *Pane, available: bool) void {
+    self.zoom.as(gtk.Widget).setVisible(if (available) 1 else 0);
+}
+
 // -------------------------------------------------------------------------
 // Drag and drop
 //
@@ -512,6 +557,10 @@ fn onWebClicked(_: *gtk.Button, self: *Pane) callconv(.c) void {
     };
 }
 
+fn onZoomClicked(_: *gtk.Button, self: *Pane) callconv(.c) void {
+    self.view.toggleZoom(self);
+}
+
 fn onCloseClicked(_: *gtk.Button, self: *Pane) callconv(.c) void {
     self.view.closePane(self);
 }

src/View.zig

@@ -65,6 +65,13 @@ panes: std.ArrayListUnmanaged(*Pane) = .empty,
 
 focused: ?*Pane = null,
 
+/// The pane temporarily filling the view, if any.
+///
+/// This is a display state and nothing more — the split tree is untouched
+/// while it is set, so leaving zoom restores the arrangement exactly rather
+/// than reconstructing it.
+zoomed: ?*Pane = null,
+
 drag: ?Drag = null,
 
 /// Set while the view is being torn down, so a pane's child exiting doesn't
@@ -119,6 +126,70 @@ pub fn widget(self: *View) *gtk.Widget {
     return self.box.as(gtk.Widget);
 }
 
+// -------------------------------------------------------------------------
+// Zoom
+//
+// One pane temporarily filling the tab, for when a split is too cramped to
+// work in. Nothing closes and nothing moves: the tree is left exactly as it
+// was and only the rendering changes, so leaving zoom restores the previous
+// arrangement rather than rebuilding an approximation of it.
+
+/// Put the view on screen. Every structural change goes through here rather
+/// than calling the layout directly, so zoom is honoured from one place
+/// instead of being re-checked at each call site.
+fn render(self: *View) void {
+    if (self.zoomed) |pane| {
+        self.layout.materializeZoom(self.box, pane);
+    } else {
+        self.layout.materialize(self.box);
+    }
+}
+
+/// Toggle `pane` filling the view.
+///
+/// A single-pane view has nothing to hide, so zooming it would be an
+/// invisible state change that leaves the button looking wrong; refuse
+/// instead.
+pub fn toggleZoom(self: *View, pane: *Pane) void {
+    if (self.zoomed == pane) {
+        self.zoomed = null;
+    } else {
+        if (self.panes.items.len < 2) return;
+        self.zoomed = pane;
+    }
+
+    self.render();
+    self.refreshZoomChrome();
+
+    // Re-rendering reparents the pane, which drops keyboard focus on the way
+    // through, so it has to be taken again afterwards.
+    pane.grabFocus();
+    self.on_title(self.ctx);
+}
+
+pub fn toggleZoomFocused(self: *View) void {
+    if (self.focusedPane()) |pane| self.toggleZoom(pane);
+}
+
+/// Leave zoom, if we are in it. Used by everything that changes the shape of
+/// the view: the point of those actions is the arrangement, and staying zoomed
+/// would hide the very thing the user just asked for.
+fn clearZoom(self: *View) void {
+    if (self.zoomed == null) return;
+    self.zoomed = null;
+    self.refreshZoomChrome();
+}
+
+/// Point every pane's zoom button at the current state: which pane (if any) is
+/// zoomed, and whether zooming means anything in a view this size.
+fn refreshZoomChrome(self: *View) void {
+    const available = self.panes.items.len > 1;
+    for (self.panes.items) |pane| {
+        pane.setZoomAvailable(available);
+        pane.setZoomed(self.zoomed == pane);
+    }
+}
+
 /// Text for the tab label: the focused pane's title, prefixed with the pane
 /// count once there is more than one.
 pub fn label(self: *View, buf: []u8) []const u8 {
@@ -172,7 +243,12 @@ pub fn addPane(self: *View, spec: Pane.Spec) !void {
     }
 
     try self.panes.append(self.alloc, pane);
-    self.layout.materialize(self.box);
+
+    // Splitting while zoomed would put the new pane somewhere you cannot see,
+    // so opening one leaves zoom.
+    self.zoomed = null;
+    self.render();
+    self.refreshZoomChrome();
 
     self.setFocused(pane);
     pane.grabFocus();
@@ -203,6 +279,11 @@ pub fn closePane(self: *View, pane: *Pane) void {
     _ = self.panes.orderedRemove(index);
     if (self.focused == pane) self.focused = null;
 
+    // The zoomed pane going away takes the zoom with it. A different pane
+    // closing — a background shell exiting, say — leaves it standing, since
+    // what you are looking at is still there and still what you asked for.
+    if (self.zoomed == pane) self.zoomed = null;
+
     // Detach the pane's widget before destroying it so materialize doesn't
     // walk into a half-freed subtree.
     if (pane.widget().getParent() != null) pane.widget().unparent();
@@ -213,7 +294,8 @@ pub fn closePane(self: *View, pane: *Pane) void {
         return;
     }
 
-    self.layout.materialize(self.box);
+    self.render();
+    self.refreshZoomChrome();
 
     const next = @min(index, self.panes.items.len - 1);
     self.setFocused(self.panes.items[next]);
@@ -283,7 +365,7 @@ pub fn applyLayout(
     const root = try self.buildNode(spec, bindings);
     self.layout.root = root;
     root.parent = null;
-    self.layout.materialize(self.box);
+    self.render();
 
     // The first pane in tree order is the top-left one, which is where you
     // would start reading the tab and so where focus belongs.
@@ -291,6 +373,10 @@ pub fn applyLayout(
         self.setFocused(self.panes.items[0]);
         self.panes.items[0].grabFocus();
     }
+
+    // Panes are built one at a time and each starts assuming it is alone, so
+    // a multi-pane layout has to be told once it is fully assembled.
+    self.refreshZoomChrome();
     self.on_title(self.ctx);
 }
 
@@ -426,7 +512,7 @@ pub fn moveTo(self: *View, moving: *Pane, target: Target, ratio: f64) void {
         (self.layout.root orelse {
             self.layout.root = node;
             node.parent = null;
-            self.layout.materialize(self.box);
+            self.render();
             return;
         });
 
@@ -435,7 +521,7 @@ pub fn moveTo(self: *View, moving: *Pane, target: Target, ratio: f64) void {
         return;
     };
 
-    self.layout.materialize(self.box);
+    self.render();
 }
 
 /// Last-resort reattachment so a pane can never be orphaned by a failed move.
@@ -451,7 +537,7 @@ fn reattachAtRoot(self: *View, node: *Layout.Node) void {
         self.layout.root = node;
         node.parent = null;
     }
-    self.layout.materialize(self.box);
+    self.render();
 }
 
 /// Move the focused pane one step in a direction: the keyboard equivalent of
@@ -460,6 +546,12 @@ pub fn moveFocused(self: *View, side: Side) void {
     const pane = self.focusedPane() orelse return;
     if (self.panes.items.len < 2) return;
 
+    // Rearranging is about where panes sit relative to each other, which is
+    // exactly what zoom is hiding. Leave it so the move can be seen — and so
+    // `neighbor` below has laid-out panes to probe for in the first place.
+    self.clearZoom();
+    self.render();
+
     const target = self.neighbor(pane, side) orelse {
         // Nothing that way, so place it against the view edge instead and let
         // it span the layout on that side.
@@ -508,6 +600,11 @@ fn neighbor(self: *View, pane: *Pane, side: Side) ?*Pane {
 pub fn beginDrag(self: *View, pane: *Pane) bool {
     if (self.panes.items.len < 2) return false;
 
+    // Only the zoomed pane is on screen, so there is nothing to drop against
+    // and no preview to show. Refusing the drag is better than starting one
+    // that can only ever be cancelled.
+    if (self.zoomed != null) return false;
+
     const node = self.layout.find(pane) orelse return false;
     const position = Layout.positionOf(node) orelse return false;
 
@@ -555,7 +652,7 @@ pub fn endDrag(self: *View) void {
         self.reattachAtRoot(node);
         return;
     };
-    self.layout.materialize(self.box);
+    self.render();
 }
 
 fn indexOf(self: *View, pane: *Pane) ?usize {

src/Window.zig

@@ -953,6 +953,10 @@ fn onShortcut(
                 if (self.activeTab()) |tab| self.beginRename(tab);
                 return 1;
             },
+            gdk.KEY_Z, gdk.KEY_z => {
+                if (self.activeTab()) |tab| tab.view.toggleZoomFocused();
+                return 1;
+            },
             gdk.KEY_V, gdk.KEY_v => {
                 // Only a terminal needs us to encode a paste for it. A web
                 // pane has its own clipboard handling, so the key is left

src/style.css

@@ -219,6 +219,17 @@
   border-color: #5b4d80;
 }
 
+/* The pane currently filling its tab. The other panes are still open and still
+   running, just not drawn, so this is marked quietly — a lit border rather than
+   anything alarming. The header's toggle icon carries the rest of the message. */
+.playpen-pane.zoomed {
+  border-color: #7a68a8;
+}
+
+.playpen-pane.zoomed .playpen-pane-button {
+  opacity: 0.55;
+}
+
 /* The pane being dragged. There is no separate drop indicator: the layout
    rearranges live during the drag, so the view itself is the preview. */
 .playpen-pane.dragging {