Tell diff renderers whether the terminal is dark or light

Renderers like delta and difftastic pick their colors for either a dark
or a light background, and they can't find out which one the terminal
has. Lazygit runs them with TERM=dumb, in a pty that doesn't answer
their queries. So the colors come from the config, and when the
terminal switches between dark and light, the diff keeps the ones it
has.

Add {{colorScheme}} to the commands of diff renderers. It is 'dark' or
'light', going by gui.colorScheme, or by the terminal if that is
'auto'. It can be passed to delta as --{{colorScheme}} and to
difftastic as --background={{colorScheme}}; other renderers can choose
between options with a template expression. When the terminal switches
between dark and light, render the diff again.

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

docs-master/Custom_DiffRenderers.md

@@ -30,21 +30,22 @@ Fields only for `rawGit`:
 The `command` of a `stdinFilter` or `extDiff` renderer is a [Go template](https://pkg.go.dev/text/template) with these variables:
 
 - `{{width}}`: the width of the view that the diff is rendered into.
+- `{{colorScheme}}`: `dark` or `light`, depending on whether the terminal has a dark or a light background. Lazygit asks the terminal about this; if yours doesn't tell, set `gui.colorScheme`.
 - `{{columnWidth}}` (only for `stdinFilter`): the width of one side of a side-by-side rendering, e.g. for `ydiff -p cat -s -w {{columnWidth}}`.
 - `{{diffContext}}` (only for `extDiff`): lazygit's current diff context size, the value controlled by the `{`/`}` keybindings.
 
-A variable can also be written with a leading dot, as in `{{.width}}`. The command can use template expressions too; for example, `delta --paging=never {{if gt .width 160}}--side-by-side{{end}}` shows the diff side by side only when there is room for it.
+A variable can also be written with a leading dot, as in `{{.width}}`. The command can use template expressions too; for example, `delta --paging=never {{if gt .width 160}}--side-by-side{{end}}` shows the diff side by side only when there is room for it, or `delta --syntax-theme={{if eq .colorScheme "light"}}Github{{else}}Dracula{{end}}` picks a different syntax theme based on the background.
 
 Here's an example for a multi-renderer setup:
 
 ```yaml
 git:
   diffRenderers:
-    - command: delta --dark --paging=never
+    - command: delta --{{colorScheme}} --paging=never
     - command: ydiff -p cat
       colorArg: never
     - type: extDiff
-      command: difft --color=always --context={{diffContext}}
+      command: difft --color=always --background={{colorScheme}} --context={{diffContext}}
     - type: rawGit
       args: [--color-words]
       name: color-words
@@ -57,12 +58,14 @@ git:
 ```yaml
 git:
   diffRenderers:
-    - command: delta --dark --paging=never
+    - command: delta --{{colorScheme}} --paging=never
 ```
 
 ![](https://i.imgur.com/QJpQkF3.png)
 
-A cool feature of delta is --hyperlinks, which renders clickable links for the line numbers in the left margin, and lazygit supports these. To use them, set the `command:` field to `delta --dark --paging=never --line-numbers --hyperlinks --hyperlinks-file-link-format="lazygit-edit://{path}:{line}"`; this allows you to click on an underlined line number in the diff to jump right to that same line in your editor.
+`--{{colorScheme}}` passes `--dark` or `--light` to delta, so that it matches the background of your terminal.
+
+A cool feature of delta is --hyperlinks, which renders clickable links for the line numbers in the left margin, and lazygit supports these. To use them, set the `command:` field to `delta --{{colorScheme}} --paging=never --line-numbers --hyperlinks --hyperlinks-file-link-format="lazygit-edit://{path}:{line}"`; this allows you to click on an underlined line number in the diff to jump right to that same line in your editor.
 
 Note that delta's `--navigate` option doesn't work in lazygit, for technical reasons.
 

pkg/config/diff_renderer_config_manager.go

@@ -67,6 +67,8 @@ type DiffRendererValues struct {
 	Width int
 	// The number of lines of context around each hunk
 	DiffContext uint64
+	// Whether the terminal has a light background
+	LightBackground bool
 }
 
 func (self *DiffRendererConfigManager) GetStdinFilterCommand(values DiffRendererValues) (string, error) {
@@ -104,8 +106,13 @@ func (self *DiffRendererConfigManager) GetExternalDiffCommand(values DiffRendere
 // the values it can refer to as its variables. A variable can be written with
 // or without the leading dot, as in {{.width}} or {{width}}.
 func (self *DiffRendererConfig) resolveCommand(values DiffRendererValues) (string, error) {
+	colorScheme := "dark"
+	if values.LightBackground {
+		colorScheme = "light"
+	}
 	variables := map[string]any{
-		"width": values.Width,
+		"width":       values.Width,
+		"colorScheme": colorScheme,
 	}
 	switch self.getType() {
 	case DiffRendererType_StdinFilter:

pkg/config/diff_renderer_config_manager_test.go

@@ -68,6 +68,7 @@ func TestGetStdinFilterCommand(t *testing.T) {
 		name               string
 		diffRendererConfig DiffRendererConfig
 		width              int
+		lightBackground    bool
 		expected           string
 		expectedError      string
 	}{
@@ -95,6 +96,26 @@ func TestGetStdinFilterCommand(t *testing.T) {
 			width:              120,
 			expected:           "delta --width=120",
 		},
+		{
+			name:               "the color scheme on a dark background",
+			diffRendererConfig: DiffRendererConfig{Command: "delta --{{colorScheme}}"},
+			width:              120,
+			expected:           "delta --dark",
+		},
+		{
+			name:               "the color scheme on a light background",
+			diffRendererConfig: DiffRendererConfig{Command: "delta --{{colorScheme}}"},
+			width:              120,
+			lightBackground:    true,
+			expected:           "delta --light",
+		},
+		{
+			name:               "the command can choose between options by the color scheme",
+			diffRendererConfig: DiffRendererConfig{Command: `delta --syntax-theme={{if eq .colorScheme "light"}}GitHub{{else}}Dracula{{end}}`},
+			width:              120,
+			lightBackground:    true,
+			expected:           "delta --syntax-theme=GitHub",
+		},
 		{
 			name:               "the command can use template expressions",
 			diffRendererConfig: DiffRendererConfig{Command: "delta{{if gt .width 100}} --side-by-side{{end}}"},
@@ -127,7 +148,7 @@ func TestGetStdinFilterCommand(t *testing.T) {
 			userConfig.Git.DiffRenderers = []DiffRendererConfig{s.diffRendererConfig}
 			config := NewDiffRendererConfigManager(func() *UserConfig { return userConfig })
 
-			command, err := config.GetStdinFilterCommand(DiffRendererValues{Width: s.width})
+			command, err := config.GetStdinFilterCommand(DiffRendererValues{Width: s.width, LightBackground: s.lightBackground})
 			if s.expectedError != "" {
 				assert.ErrorContains(t, err, s.expectedError)
 			} else {
@@ -155,6 +176,11 @@ func TestGetExternalDiffCommand(t *testing.T) {
 			diffRendererConfig: DiffRendererConfig{Type: "extDiff", Command: "difft --width={{width}}"},
 			expected:           "difft --width=120",
 		},
+		{
+			name:               "the color scheme",
+			diffRendererConfig: DiffRendererConfig{Type: "extDiff", Command: "difft --background={{colorScheme}}"},
+			expected:           "difft --background=dark",
+		},
 		{
 			name:               "the width alongside the diff context size",
 			diffRendererConfig: DiffRendererConfig{Type: "extDiff", Command: "difft --width={{width}} --context={{diffContext}}"},

pkg/gui/gui.go

@@ -948,6 +948,7 @@ func (gui *Gui) Run(startArgs appTypes.StartArgs) error {
 		gui.applyTerminalBackground()
 		gui.c.Contexts().LocalCommits.HandleRender()
 		gui.c.Contexts().SubCommits.HandleRender()
+		gui.helpers.Diff.RenderToMainAgain()
 		return nil
 	})
 

pkg/gui/main_view_render.go

@@ -57,7 +57,11 @@ func (gui *Gui) newRenderTask(view *gocui.View, cmd *exec.Cmd, prefix string) er
 		// width to render at known, and with it the renderer command.
 		width := view.InnerWidth()
 		diffRendererConfigManager := gui.stateAccessor.GetDiffRendererConfigManager()
-		values := config.DiffRendererValues{Width: width, DiffContext: gui.c.UserConfig().Git.DiffContextSize}
+		values := config.DiffRendererValues{
+			Width:           width,
+			DiffContext:     gui.c.UserConfig().Git.DiffContextSize,
+			LightBackground: gui.terminalHasLightBackground(),
+		}
 		stdinFilter, stdinFilterErr := diffRendererConfigManager.GetStdinFilterCommand(values)
 		externalDiff, externalDiffErr := diffRendererConfigManager.GetExternalDiffCommand(values)
 		if err := errors.Join(stdinFilterErr, externalDiffErr); err != nil {