tests: Add regression for v1.13.1 pane spacing issue

tests/fixtures/regressions/issue_800_default_size_many_windows.yaml

@@ -0,0 +1,12 @@
+session_name: many-windows-issue
+windows:
+- window_name: moo
+  layout: main-horizontal
+  panes:
+  - echo hello
+  - echo hello
+  - echo hello
+  - echo hello
+  - echo hello
+  - echo hello
+  - echo hello

tests/test_workspacebuilder.py

@@ -1217,3 +1217,29 @@ def is_almost_equal(x, y):
 
     assert is_almost_equal(height(panes[0]), height(panes[1]))
     assert is_almost_equal(width(panes[0]), width(panes[1]))
+
+
+def test_issue_800_default_size_many_windows(server, monkeypatch):
+    yaml_config = test_utils.read_config_file(
+        "regressions/issue_800_default_size_many_windows.yaml"
+    )
+    sconfig = kaptan.Kaptan(handler="yaml")
+    sconfig = sconfig.import_config(yaml_config).get()
+    sconfig = config.expand(sconfig)
+    sconfig = config.trickle(sconfig)
+
+    builder = WorkspaceBuilder(sconf=sconfig, server=server)
+    builder.build()
+
+    assert len(server.list_sessions()) == 1
+
+    # Assign an active pane to the session
+    second_session = server.list_sessions()[0]
+    first_pane_on_second_session_id = second_session.list_windows()[0].list_panes()[0][
+        "pane_id"
+    ]
+    monkeypatch.setenv("TMUX_PANE", first_pane_on_second_session_id)
+
+    builder = WorkspaceBuilder(sconf=sconfig, server=server)
+
+    assert builder.find_current_attached_session() == second_session