Classify which side panels show a diff in their main view

The focused main view is about to show a selection, but only where there
are diff lines to act on: a branch's commit log or the status dashboard
has nothing to select. Rather than have the main view guess from the
rendered content, let the side panels say so, since each of them knows
what it renders.

The classification is finer than a yes/no because acting on a selection
means different things per panel — staging into the working tree for the
files panel, taking lines into a custom patch for the commit panels — and
those actions want the same one answer as this. Only whether a panel
shows a diff at all is read for now.

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

pkg/gui/context/commit_files_context.go

@@ -19,11 +19,16 @@ type CommitFilesContext struct {
 }
 
 var (
-	_ types.IListContext       = (*CommitFilesContext)(nil)
-	_ types.DiffableContext    = (*CommitFilesContext)(nil)
-	_ types.IFilterableContext = (*CommitFilesContext)(nil)
+	_ types.IListContext        = (*CommitFilesContext)(nil)
+	_ types.DiffableContext     = (*CommitFilesContext)(nil)
+	_ types.IFilterableContext  = (*CommitFilesContext)(nil)
+	_ types.DiffMainViewContext = (*CommitFilesContext)(nil)
 )
 
+func (self *CommitFilesContext) GetDiffMainViewType() types.DiffMainViewType {
+	return types.DiffMainViewTypePatchBuilding
+}
+
 func NewCommitFilesContext(c *ContextCommon) *CommitFilesContext {
 	viewModel := filetree.NewCommitFileTreeViewModel(
 		func() []*models.CommitFile { return c.Model().CommitFiles },

pkg/gui/context/local_commits_context.go

@@ -32,11 +32,16 @@ type commitDropIndicator struct {
 }
 
 var (
-	_ types.IListContext       = (*LocalCommitsContext)(nil)
-	_ types.DiffableContext    = (*LocalCommitsContext)(nil)
-	_ types.ISearchableContext = (*LocalCommitsContext)(nil)
+	_ types.IListContext        = (*LocalCommitsContext)(nil)
+	_ types.DiffableContext     = (*LocalCommitsContext)(nil)
+	_ types.ISearchableContext  = (*LocalCommitsContext)(nil)
+	_ types.DiffMainViewContext = (*LocalCommitsContext)(nil)
 )
 
+func (self *LocalCommitsContext) GetDiffMainViewType() types.DiffMainViewType {
+	return types.DiffMainViewTypePatchBuilding
+}
+
 func NewLocalCommitsContext(c *ContextCommon) *LocalCommitsContext {
 	dropIndicator := &commitDropIndicator{insertionIndex: -1}
 	viewModel := NewLocalCommitsViewModel(

pkg/gui/context/reflog_commits_context.go

@@ -14,10 +14,15 @@ type ReflogCommitsContext struct {
 }
 
 var (
-	_ types.IListContext    = (*ReflogCommitsContext)(nil)
-	_ types.DiffableContext = (*ReflogCommitsContext)(nil)
+	_ types.IListContext        = (*ReflogCommitsContext)(nil)
+	_ types.DiffableContext     = (*ReflogCommitsContext)(nil)
+	_ types.DiffMainViewContext = (*ReflogCommitsContext)(nil)
 )
 
+func (self *ReflogCommitsContext) GetDiffMainViewType() types.DiffMainViewType {
+	return types.DiffMainViewTypePatchBuilding
+}
+
 func NewReflogCommitsContext(c *ContextCommon) *ReflogCommitsContext {
 	viewModel := NewFilteredListViewModel(
 		func() []*models.Commit { return c.Model().FilteredReflogCommits },

pkg/gui/context/stash_context.go

@@ -12,10 +12,15 @@ type StashContext struct {
 }
 
 var (
-	_ types.IListContext    = (*StashContext)(nil)
-	_ types.DiffableContext = (*StashContext)(nil)
+	_ types.IListContext        = (*StashContext)(nil)
+	_ types.DiffableContext     = (*StashContext)(nil)
+	_ types.DiffMainViewContext = (*StashContext)(nil)
 )
 
+func (self *StashContext) GetDiffMainViewType() types.DiffMainViewType {
+	return types.DiffMainViewTypePatchBuilding
+}
+
 func NewStashContext(
 	c *ContextCommon,
 ) *StashContext {

pkg/gui/context/sub_commits_context.go

@@ -21,11 +21,16 @@ type SubCommitsContext struct {
 }
 
 var (
-	_ types.IListContext       = (*SubCommitsContext)(nil)
-	_ types.DiffableContext    = (*SubCommitsContext)(nil)
-	_ types.ISearchableContext = (*SubCommitsContext)(nil)
+	_ types.IListContext        = (*SubCommitsContext)(nil)
+	_ types.DiffableContext     = (*SubCommitsContext)(nil)
+	_ types.ISearchableContext  = (*SubCommitsContext)(nil)
+	_ types.DiffMainViewContext = (*SubCommitsContext)(nil)
 )
 
+func (self *SubCommitsContext) GetDiffMainViewType() types.DiffMainViewType {
+	return types.DiffMainViewTypePatchBuilding
+}
+
 func NewSubCommitsContext(
 	c *ContextCommon,
 ) *SubCommitsContext {

pkg/gui/context/working_tree_context.go

@@ -15,10 +15,15 @@ type WorkingTreeContext struct {
 }
 
 var (
-	_ types.IListContext       = (*WorkingTreeContext)(nil)
-	_ types.IFilterableContext = (*WorkingTreeContext)(nil)
+	_ types.IListContext        = (*WorkingTreeContext)(nil)
+	_ types.IFilterableContext  = (*WorkingTreeContext)(nil)
+	_ types.DiffMainViewContext = (*WorkingTreeContext)(nil)
 )
 
+func (self *WorkingTreeContext) GetDiffMainViewType() types.DiffMainViewType {
+	return types.DiffMainViewTypeStaging
+}
+
 func NewWorkingTreeContext(c *ContextCommon) *WorkingTreeContext {
 	viewModel := filetree.NewFileTreeViewModel(
 		func() []*models.File { return c.Model().Files },

pkg/gui/types/context.go

@@ -180,6 +180,39 @@ type DiffableContext interface {
 	RefForAdjustingLineNumberInDiff() string
 }
 
+// DiffMainViewContext is implemented by the side panel contexts whose focused
+// main view shows a unified diff — files, local commits, sub-commits, reflog,
+// stash, and commit files — as opposed to a commit log or other non-diff content
+// (branches, tags, status, …). It is distinct from DiffableContext, which is
+// about producing a diff between two refs for the diff menu. The focused main
+// view shows a selection only for a context that implements this: a selection is
+// only meaningful where there are diff lines to act on (edit one, copy some, jump
+// by hunk or file). The returned type additionally classifies what acting on that
+// selection means.
+type DiffMainViewContext interface {
+	Context
+
+	GetDiffMainViewType() DiffMainViewType
+}
+
+// DiffMainViewType classifies what the focused main view's diff belongs to, which
+// decides what acting on a selection in it means.
+type DiffMainViewType int
+
+const (
+	// DiffMainViewTypeNone: the main view holds no diff, so there is nothing to
+	// select. A side panel that doesn't implement DiffMainViewContext counts as
+	// this; no panel returns it itself.
+	DiffMainViewTypeNone DiffMainViewType = iota
+	// DiffMainViewTypeStaging: the diff is the working tree's, so the selection can
+	// be staged or unstaged (the files panel).
+	DiffMainViewTypeStaging
+	// DiffMainViewTypePatchBuilding: the diff belongs to a commit, so the selection
+	// can be taken into a custom patch (the commit files / commits / sub-commits /
+	// reflog / stash panels).
+	DiffMainViewTypePatchBuilding
+)
+
 type IListContext interface {
 	Context