diff: an attempt to open gives way once the terminal loses room

The room only fed the decision to open: an attempt whose read found the
terminal narrowed went on reading for each edit or command that landed
during a read, with no room to open anything. It now ends there, and the
next edit with room starts an attempt of its own.

The README's `git status` sentence says where that read now happens: at
the first fetch, the one an edit decides on included.

mods/diff/README.md

@@ -58,8 +58,8 @@ forgets it); and the working tree is read only by a fetch for a pane that
 is open, after an edit that landed or a shell command that ran, or by the
 fetch each edit inside the repository, with room for a pane, makes until
 one lists a file and the pane opens on it. The one read the built-in has
-no counterpart for is a `git status` at a pane's first fetch, which stands
-in for the change time the built-in dates a moved file by.
+no counterpart for is a `git status` at the first of those fetches, which
+stands in for the change time the built-in dates a moved file by.
 
 `hooks/register.ts` is the module; everything under `hooks/` is its parts.
 

mods/diff/hooks/register.ts

@@ -514,7 +514,9 @@ export function register(on: On) {
           }
         }
 
-        if (seen === landed || isOvertaken) {
+        const hasLostRoom = !isListing && !hasRoomFor(floor)
+
+        if (seen === landed || isOvertaken || hasLostRoom) {
           return
         }
 

mods/diff/tests/register.test.ts

@@ -875,6 +875,44 @@ describe('register', () => {
     expect(world.opened, 'the room it read for is gone').toEqual([])
   })
 
+  test('nor does a command landing in that read have it read again, the next edit with room does', async ($, on) => {
+    const world = Fixtures.inSlowRepository(on)
+
+    const edit = () =>
+      $.tool.call({
+        tool: 'Edit',
+        file_path: '/work/app.ts',
+        old_string: '1',
+        new_string: '2',
+      })
+
+    on('tool.call', () => ({ result: 'done' }))
+
+    await $.session.start(Fixtures.SESSION)
+    await $.ui.render(Fixtures.HINT)
+    await edit()
+    await world.clock.settle()
+    await $.ui.render(Fixtures.hintAt(Limits.AUTO_OPEN_MIN_COLUMNS - 1))
+    await $.tool.call({ tool: 'Bash', command: 'make' })
+    await world.clock.advance(Fixtures.SETTLE_MS)
+
+    expect(
+      world.reads,
+      'the attempt gave way with the room; it read for nothing more',
+    ).toEqual([0])
+
+    expect(world.opened).toEqual([])
+
+    await $.ui.render(Fixtures.HINT)
+    await edit()
+    await world.clock.advance(Fixtures.SETTLE_MS)
+
+    expect(
+      world.opened.map(pane => pane.id),
+      'wide again, the next edit starts an attempt of its own',
+    ).toEqual(['diff'])
+  })
+
   test("only the main loop's edit opens the pane", async ($, on) => {
     const world = Fixtures.inRepository(on)