Add DisabledReason field to MenuItem

This is useful to disable items that are not applicable right now because of
some condition (e.g. the "delete branch" menu item when the currently
checked-out branch is selected).

When a DisabledReason is set on a menu item, we
- show it in a tooltip (below the regular tooltip of the item, if it has one)
- strike through the item's key, if it has one
- show an error message with the DisabledReason if the user tries to invoke the
  command

pkg/gui/context/menu_context.go

@@ -90,6 +90,9 @@ func (self *MenuViewModel) GetDisplayStrings(_ int, _ int) [][]string {
 
 	return lo.Map(menuItems, func(item *types.MenuItem, _ int) []string {
 		displayStrings := item.LabelColumns
+		if item.DisabledReason != "" {
+			displayStrings[0] = style.FgDefault.SetStrikethrough().Sprint(displayStrings[0])
+		}
 
 		if !showKeys {
 			return displayStrings
@@ -169,6 +172,10 @@ func (self *MenuContext) GetKeybindings(opts types.KeybindingsOpts) []*types.Bin
 }
 
 func (self *MenuContext) OnMenuPress(selectedItem *types.MenuItem) error {
+	if selectedItem != nil && selectedItem.DisabledReason != "" {
+		return self.c.ErrorMsg(selectedItem.DisabledReason)
+	}
+
 	if err := self.c.PopContext(); err != nil {
 		return err
 	}

pkg/gui/controllers/helpers/confirmation_helper.go

@@ -333,7 +333,7 @@ func (self *ConfirmationHelper) resizeMenu() {
 	tooltip := ""
 	selectedItem := self.c.Contexts().Menu.GetSelected()
 	if selectedItem != nil {
-		tooltip = selectedItem.Tooltip
+		tooltip = self.TooltipForMenuItem(selectedItem)
 	}
 	tooltipHeight := getMessageHeight(true, tooltip, panelWidth) + 2 // plus 2 for the frame
 	_, _ = self.c.GocuiGui().SetView(self.c.Views().Tooltip.Name(), x0, tooltipTop, x1, tooltipTop+tooltipHeight-1, 0)
@@ -382,3 +382,14 @@ func (self *ConfirmationHelper) IsPopupPanel(viewName string) bool {
 func (self *ConfirmationHelper) IsPopupPanelFocused() bool {
 	return self.IsPopupPanel(self.c.CurrentContext().GetViewName())
 }
+
+func (self *ConfirmationHelper) TooltipForMenuItem(menuItem *types.MenuItem) string {
+	tooltip := menuItem.Tooltip
+	if menuItem.DisabledReason != "" {
+		if tooltip != "" {
+			tooltip += "\n\n"
+		}
+		tooltip += style.FgRed.Sprintf(self.c.Tr.DisabledMenuItemPrefix) + menuItem.DisabledReason
+	}
+	return tooltip
+}

pkg/gui/controllers/menu_controller.go

@@ -54,7 +54,7 @@ func (self *MenuController) GetOnFocus() func(types.OnFocusOpts) error {
 	return func(types.OnFocusOpts) error {
 		selectedMenuItem := self.context().GetSelected()
 		if selectedMenuItem != nil {
-			self.c.Views().Tooltip.SetContent(selectedMenuItem.Tooltip)
+			self.c.Views().Tooltip.SetContent(self.c.Helpers().Confirmation.TooltipForMenuItem(selectedMenuItem))
 		}
 		return nil
 	}

pkg/gui/types/common.go

@@ -204,6 +204,10 @@ type MenuItem struct {
 	// The tooltip will be displayed upon highlighting the menu item
 	Tooltip string
 
+	// If non-empty, show this in a tooltip, style the menu item as disabled,
+	// and refuse to invoke the command
+	DisabledReason string
+
 	// Can be used to group menu items into sections with headers. MenuItems
 	// with the same Section should be contiguous, and will automatically get a
 	// section header. If nil, the item is not part of a section.

pkg/i18n/english.go

@@ -612,6 +612,7 @@ type TranslationSet struct {
 	MarkAsBaseCommitTooltip             string
 	MarkedCommitMarker                  string
 	PleaseGoToURL                       string
+	DisabledMenuItemPrefix              string
 	Actions                             Actions
 	Bisect                              Bisect
 	Log                                 Log
@@ -1403,6 +1404,7 @@ func EnglishTranslationSet() TranslationSet {
 		MarkAsBaseCommitTooltip:             "Select a base commit for the next rebase; this will effectively perform a 'git rebase --onto'.",
 		MarkedCommitMarker:                  "↑↑↑ Will rebase from here ↑↑↑",
 		PleaseGoToURL:                       "Please go to {{.url}}",
+		DisabledMenuItemPrefix:              "Disabled: ",
 		Actions: Actions{
 			// TODO: combine this with the original keybinding descriptions (those are all in lowercase atm)
 			CheckoutCommit:                    "Checkout commit",