Allow overriding the platform used for default keybindings (#5671)

A handful of default keybindings differ by platform (e.g. word-wise
cursor movement in text inputs uses alt on macOS but ctrl elsewhere).
Lazygit chooses these based on the OS it runs on, but that's the wrong
signal when the OS isn't where the user is actually typing: someone
running lazygit in a Linux container that they access over ssh from a
Mac gets the Linux bindings, when they'd rather have the Mac ones.
Remapping each binding by hand via config is tedious, so add a single
LAZYGIT_KEYBINDING_PLATFORM override.

Closes #5668.

docs-master/Config.md

@@ -1102,6 +1102,12 @@ keybinding:
     edit: <disabled> # disable 'edit file'
 ```
 
+### Overriding the platform for default keybindings
+
+A few keybindings have different defaults on macOS than on Linux and Windows (e.g. word-wise cursor movement in text inputs uses `alt` on macOS but `ctrl` elsewhere). Lazygit picks these based on the OS it's running on, but you can override that with the `LAZYGIT_KEYBINDING_PLATFORM` environment variable. Set it to `darwin`, `linux`, or `windows`; any other value is ignored and the actual OS is used.
+
+This is useful when running lazygit in a Linux container that you access over ssh from a Mac, where you'd rather use the macOS keybindings.
+
 ### Example Keybindings For Colemak Users
 
 ```yaml

pkg/app/entry_point.go

@@ -102,7 +102,7 @@ func Start(buildInfo *BuildInfo, integrationTest integrationTypes.IntegrationTes
 	if cliArgs.PrintDefaultConfig {
 		var buf bytes.Buffer
 		encoder := yaml.NewEncoder(&buf)
-		err := encoder.Encode(config.GetDefaultConfigForPlatform(runtime.GOOS))
+		err := encoder.Encode(config.GetDefaultConfigForPlatform(config.KeybindingPlatform()))
 		if err != nil {
 			log.Fatal(err.Error())
 		}

pkg/config/app_config.go

@@ -136,8 +136,23 @@ func findOrCreateConfigDir() (string, error) {
 	return folder, os.MkdirAll(folder, 0o755)
 }
 
+// KeybindingPlatform returns the platform whose default keybindings should be
+// used. Normally this is the OS we're running on, but it can be overridden with
+// the LAZYGIT_KEYBINDING_PLATFORM environment variable; this is useful e.g. when
+// running lazygit in a Linux container that you access over ssh from a Mac, and
+// you'd rather use the Mac keybindings. An unrecognized value falls back to the
+// real OS, which gives meaningful bindings, rather than to the (arbitrary)
+// non-darwin defaults.
+func KeybindingPlatform() string {
+	platform := os.Getenv("LAZYGIT_KEYBINDING_PLATFORM")
+	if lo.Contains([]string{"darwin", "linux", "windows"}, platform) {
+		return platform
+	}
+	return runtime.GOOS
+}
+
 func loadUserConfigWithDefaults(configFiles []*ConfigFile, isGuiInitialized bool) (*UserConfig, error) {
-	return loadUserConfig(configFiles, GetDefaultConfigForPlatform(runtime.GOOS), isGuiInitialized)
+	return loadUserConfig(configFiles, GetDefaultConfigForPlatform(KeybindingPlatform()), isGuiInitialized)
 }
 
 func loadUserConfig(configFiles []*ConfigFile, base *UserConfig, isGuiInitialized bool) (*UserConfig, error) {

pkg/config/app_config_test.go

@@ -1,11 +1,53 @@
 package config
 
 import (
+	"runtime"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
 )
 
+func TestKeybindingPlatform(t *testing.T) {
+	scenarios := []struct {
+		name     string
+		envValue string
+		expected string
+	}{
+		{
+			name:     "Not set falls back to the host OS",
+			envValue: "",
+			expected: runtime.GOOS,
+		},
+		{
+			name:     "darwin is honored",
+			envValue: "darwin",
+			expected: "darwin",
+		},
+		{
+			name:     "linux is honored",
+			envValue: "linux",
+			expected: "linux",
+		},
+		{
+			name:     "windows is honored",
+			envValue: "windows",
+			expected: "windows",
+		},
+		{
+			name:     "An unrecognized value falls back to the host OS",
+			envValue: "mac",
+			expected: runtime.GOOS,
+		},
+	}
+
+	for _, s := range scenarios {
+		t.Run(s.name, func(t *testing.T) {
+			t.Setenv("LAZYGIT_KEYBINDING_PLATFORM", s.envValue)
+			assert.Equal(t, s.expected, KeybindingPlatform())
+		})
+	}
+}
+
 func TestMigrationOfRenamedKeys(t *testing.T) {
 	scenarios := []struct {
 		name              string