Let an external diff command be told the width it renders at

Like a stdin filter, an external diff program lays out its rendering to
the width it reads off the terminal, and a render through a pipe leaves
it nothing to read. difftastic in side-by-side mode is the case that
shows it.

Offer the width as the {{width}} template variable, the same name the
stdin filter command takes it by. Since difftastic supports the COLUMNS
variable, which we will set later in this branch, and I don't know of
any other external-diff renderer that doesn't, we don't document this.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

pkg/config/diff_renderer_config_manager.go

@@ -88,7 +88,7 @@ func (self *DiffRendererConfigManager) GetColorArg() string {
 	return colorArg
 }
 
-func (self *DiffRendererConfigManager) GetExternalDiffCommand(diffContext uint64) string {
+func (self *DiffRendererConfigManager) GetExternalDiffCommand(diffContext uint64, width int) string {
 	currentDiffRendererConfig := self.currentDiffRendererConfig()
 	if currentDiffRendererConfig == nil || currentDiffRendererConfig.getType() != DiffRendererType_ExtDiff {
 		return ""
@@ -96,6 +96,7 @@ func (self *DiffRendererConfigManager) GetExternalDiffCommand(diffContext uint64
 
 	templateValues := map[string]string{
 		"diffContext": strconv.Itoa(int(diffContext)),
+		"width":       strconv.Itoa(width),
 	}
 
 	return utils.ResolvePlaceholderString(string(currentDiffRendererConfig.Command), templateValues)

pkg/config/diff_renderer_config_manager_test.go

@@ -113,6 +113,45 @@ func TestGetStdinFilterCommand(t *testing.T) {
 	}
 }
 
+func TestGetExternalDiffCommand(t *testing.T) {
+	scenarios := []struct {
+		name               string
+		diffRendererConfig DiffRendererConfig
+		expected           string
+	}{
+		{
+			name:               "a command without template variables is passed through",
+			diffRendererConfig: DiffRendererConfig{Type: "extDiff", Command: "difft --color=always"},
+			expected:           "difft --color=always",
+		},
+		{
+			name:               "the width the diff is rendered at",
+			diffRendererConfig: DiffRendererConfig{Type: "extDiff", Command: "difft --width={{width}}"},
+			expected:           "difft --width=120",
+		},
+		{
+			name:               "the width alongside the diff context size",
+			diffRendererConfig: DiffRendererConfig{Type: "extDiff", Command: "difft --width={{width}} --context={{diffContext}}"},
+			expected:           "difft --width=120 --context=3",
+		},
+		{
+			name:               "nothing is returned for a renderer of another type",
+			diffRendererConfig: DiffRendererConfig{Command: "delta --width={{width}}"},
+			expected:           "",
+		},
+	}
+
+	for _, s := range scenarios {
+		t.Run(s.name, func(t *testing.T) {
+			userConfig := &UserConfig{}
+			userConfig.Git.DiffRenderers = []DiffRendererConfig{s.diffRendererConfig}
+			config := NewDiffRendererConfigManager(func() *UserConfig { return userConfig })
+
+			assert.Equal(t, s.expected, config.GetExternalDiffCommand(3, 120))
+		})
+	}
+}
+
 func TestCurrentDiffRendererNameWithoutDiffRenderers(t *testing.T) {
 	config := NewDiffRendererConfigManager(func() *UserConfig { return &UserConfig{} })
 

pkg/gui/pty.go

@@ -89,7 +89,7 @@ func (gui *Gui) newPtyTask(view *gocui.View, cmd *exec.Cmd, prefix string) error
 		width = view.InnerWidth()
 		diffRendererConfigManager := gui.stateAccessor.GetDiffRendererConfigManager()
 		pager := diffRendererConfigManager.GetStdinFilterCommand(width)
-		externalDiff := diffRendererConfigManager.GetExternalDiffCommand(gui.c.UserConfig().Git.DiffContextSize)
+		externalDiff := diffRendererConfigManager.GetExternalDiffCommand(gui.c.UserConfig().Git.DiffContextSize, width)
 
 		cmdStr := strings.Join(cmd.Args, " ")