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