fix(client-runtime): terminals fit their pane again after a reconnect

packages/client-runtime/src/state/terminalSession.test.ts

@@ -245,10 +245,14 @@ describe("terminal session reducers", () => {
       snapshot: BASE_SNAPSHOT,
     });
     let cursor = readTerminalOutputUpdate(state.output, INITIAL_TERMINAL_OUTPUT_CURSOR).cursor;
+    const attachedGeneration = state.output.generation;
+    // A socket reconnect resubscribes the same stream, so the server's new snapshot gets no seed.
     state = applyTerminalAttachStreamEvent(state, { type: "snapshot", snapshot: BASE_SNAPSHOT });
     const repeated = readTerminalOutputUpdate(state.output, cursor);
     expect(repeated).toMatchObject({ type: "reset", data: "hello" });
     expect(state.version).toBe(2);
+    // Viewers replay their measured grid once per generation; the new attachment holds the old size.
+    expect(state.output.generation).not.toBe(attachedGeneration);
     cursor = repeated.cursor;
 
     state = applyTerminalAttachStreamEvent(state, {

packages/client-runtime/src/state/terminalSession.ts

@@ -162,12 +162,21 @@ export function applyTerminalAttachStreamEvent(
   maxBufferBytes = DEFAULT_MAX_TERMINAL_BUFFER_BYTES,
 ): TerminalBufferState {
   switch (event.type) {
-    case "snapshot":
+    case "snapshot": {
+      if (current.version === 0) {
+        return terminalBufferStateFromSnapshot(event.snapshot, maxBufferBytes, current);
+      }
+      // A later snapshot is a new server attachment, as after a socket reconnect. It starts a
+      // new generation because viewers replay their measured grid once per generation.
+      const reattached = {
+        ...current,
+        output: { ...current.output, generation: ++terminalAttachGeneration },
+      };
       return {
-        ...terminalBufferStateFromSnapshot(event.snapshot, maxBufferBytes, current),
-        lifecycleVersion:
-          current.version === 0 ? current.lifecycleVersion : current.lifecycleVersion + 1,
+        ...terminalBufferStateFromSnapshot(event.snapshot, maxBufferBytes, reattached),
+        lifecycleVersion: current.lifecycleVersion + 1,
       };
+    }
     case "restarted":
       return {
         ...terminalBufferStateFromSnapshot(event.snapshot, maxBufferBytes, current),