Add ability to restore previous layout.

README.md

@@ -441,6 +441,81 @@ GPU. Here, each frame walks the visible rows, groups cells into runs of
 identical style, and hands each run to Pango. That is far more work per frame
 in principle, but a terminal grid is small.
 
+## Restoring a session
+
+Quitting in the middle of something should not cost you the arrangement you were
+in the middle of. Every exit writes a **session snapshot** — a photograph of the
+window as it stood — and the next launch that finds one puts a banner at the foot
+of the sidebar: *Restore session*, and how many tabs are in it. Click it and
+those tabs come back; press the ✕ and it goes away. Doing nothing is the same as
+dismissing it.
+
+This is the counterpart to the [startup list](#startup-tabs), not a replacement
+for it, and the difference is the whole point. A startup entry is a **recipe** you
+maintain: open this layout against these values, written down once because you
+want it every morning. A snapshot is a **photograph** nobody asked for, of
+wherever the tabs had actually got to. A recipe cannot describe a shell that has
+been `cd`-ed three directories deep, and a photograph is not something you would
+ever sit down and edit — so the app keeps both.
+
+What is in the photograph is each tab's split tree: the shape, the divider
+ratios, every terminal's working directory, every web pane's URL, the repository
+a review was bound to, and the name and emoji the row was wearing. What is
+deliberately not in it is anything that was *running* — scrollback, shell
+history, a half-typed command, the processes themselves. Restoring puts the
+arrangement back and leaves the prompts empty. That is the honest version of the
+feature: a snapshot that claimed to bring a build back would be lying about the
+first thing you would check.
+
+Restoring **replaces** the tabs the window opened for itself, rather than adding
+to them — a window holding both your startup tabs and the session they were
+standing in for is two of everything. That is also why the offer only stands
+until the tab set changes: open or close a tab yourself and the banner goes,
+because "the tabs the window opened" has stopped being a set anyone can point at,
+and closing it would be closing your work. The banner is the launch-time gesture
+it looks like.
+
+Closing every tab by hand takes the snapshot with it. That route out is a
+deliberate one — there is nothing left worth offering to put back, and a banner
+at the next launch offering the session before that would be answering a question
+nobody asked.
+
+The file is `session.json`, and it lives in the **state** directory rather than
+beside `layouts.json` and `settings.json`:
+
+```
+~/.local/state/playpen/session.json     ($XDG_STATE_HOME/playpen/session.json)
+```
+
+Those two are files a person writes; this one is written behind your back on
+every exit, and putting that much churn in a config directory — which plenty of
+people keep in version control — would make every quit look like an edit. Its
+per-tab `root` is the same split-tree grammar a saved layout's is, so the two can
+never drift into dialects of one shape, with the difference that a snapshot is
+taken *after* parameter substitution: every path in it is literal, because it is
+the directory a shell was really sitting in.
+
+```json
+{
+  "version": 1,
+  "tabs": [
+    { "name": "signal", "emoji": "🚀", "layout": "Project",
+      "root": {
+        "split": "horizontal", "ratio": 0.35,
+        "first":  { "kind": "terminal", "cwd": "/home/you/src/signal" },
+        "second": { "kind": "review",   "cwd": "/home/you/src/signal" }
+      } },
+    { "name": "scratch",
+      "root": { "kind": "terminal", "cwd": "/home/you" } }
+  ]
+}
+```
+
+The `layout` a tab came from rides along even though it is not what reopens it.
+It is there so that a restored window can still be captured by *Use current
+tabs* — without it, restoring a session and then asking to keep those tabs at
+launch would quietly write down a window of plain shells.
+
 ## Quitting
 
 Closing the window asks first, and does so by default. A window here is not one
@@ -509,6 +584,11 @@ to closing the moment the window manager says so:
 - **A startup list**: the tabs to open at launch, each a layout with its
   parameters filled in, fillable from the tabs you have open now. See
   [Startup tabs](#startup-tabs)
+- **A session to pick up where you left off**: every exit photographs the
+  window, and the next launch offers it back from a banner at the foot of the
+  sidebar — click to reopen those tabs, or dismiss it and carry on. Unlike the
+  startup list this is the arrangement you actually had, directories and all.
+  See [Restoring a session](#restoring-a-session)
 - **Pane status in the tab strip**, driven by OSC 9;4, so a tab can say whether
   it is working, waiting on you, or finished and still unanswered. See
   [Agent status](#agent-status)