Add a test for emptying a pane before its diff's task is created

If the lower pane of the main view is asked to show a diff and is then
emptied before the next layout, it ends up holding the diff. Selecting
a file with staged changes and moving back to one without any in rapid
succession does this. The pane is hidden then, but when it is shown
again, it shows the other file's diff until its next render replaces
it.

Emptying a pane clears the view right away, but leaves the view's tasks
alone. So the diff's task, which is only created after the layout,
fills the pane again. A task that is still reading a diff into the pane
isn't stopped either, so it can write into the emptied view.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

pkg/integration/tests/main_view/keep_an_emptied_pane_empty.go

@@ -0,0 +1,46 @@
+package main_view
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+// Both selection changes are handled before the next layout. Selecting file_a asks
+// for its staged changes in the lower pane, whose task is only created after the
+// layout. Selecting file_b again leaves that pane with nothing to show, and empties it
+// right away. The pane is hidden then, but when it is shown again, it shows what it
+// holds until its next render replaces it.
+var KeepAnEmptiedPaneEmpty = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Selecting a file and moving back off it in rapid succession leaves the pane that the file filled empty",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig:  func(config *config.AppConfig) {},
+	SetupRepo: func(shell *Shell) {
+		shell.CreateFileAndAdd("file_a", "one\n")
+		shell.CreateFileAndAdd("file_b", "one\n")
+		shell.Commit("one")
+
+		shell.UpdateFileAndAdd("file_a", "STAGED\n")
+		shell.UpdateFile("file_a", "UNSTAGED\n")
+		shell.UpdateFile("file_b", "two\n")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Files().
+			IsFocused().
+			NavigateToLine(Contains("file_b"))
+
+		t.Views().Secondary().
+			IsInvisible()
+
+		t.Views().Files().
+			PressRapidly(keys.Universal.PrevItem, keys.Universal.NextItem).
+			SelectedLine(Contains("file_b"))
+
+		t.Views().Secondary().
+			IsInvisible().
+			/* EXPECTED:
+			Content(Equals(""))
+			ACTUAL: */
+			Content(Contains("+STAGED"))
+	},
+})

pkg/integration/tests/test_list.go

@@ -410,6 +410,7 @@ var tests = []*components.IntegrationTest{
 	main_view.JumpToAFileOfTheDiff,
 	main_view.JumpToAFileOnlyOverADiff,
 	main_view.KeepAWrappedLineCoveredAcrossARerender,
+	main_view.KeepAnEmptiedPaneEmpty,
 	main_view.KeepBothHalvesOfAChangeSelected,
 	main_view.KeepPositionByTheVisibleEndOfASelection,
 	main_view.KeepPositionInBothPanesWhenChangingContextSize,