feat(shell): support vimode segment in fish

Extend the existing `vimode` segment to fish. When a config contains a
`vimode` segment and the active shell is fish, Oh My Posh enables an
`_omp_enable_vimode` hook that registers an `--on-variable fish_bind_mode`
handler. The handler mirrors fish's bind mode into `POSH_VI_MODE` (translated
to the keymap names the segment already understands: `default` -> `vicmd`,
`insert` -> `viins`, `replace`/`replace_one` -> `replace`, `visual` passthrough)
and re-renders the prompt on every mode change.

The hook only activates when vi or hybrid key bindings are in use, and it
overrides fish's built-in `fish_mode_prompt` so the mode is shown solely by the
segment. The segment implementation itself is unchanged.

Related #5438

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/config/config.go

@@ -211,7 +211,7 @@ func (cfg *Config) Features(env runtime.Environment) shell.Features {
 				}
 			}
 
-			if segment.Type == VIMODE && slices.Contains([]string{shell.ZSH, shell.PWSH}, env.Shell()) {
+			if segment.Type == VIMODE && slices.Contains([]string{shell.ZSH, shell.PWSH, shell.FISH}, env.Shell()) {
 				log.Debug("vi mode tracking enabled")
 				feats |= shell.VIMode
 			}

src/config/config_test.go

@@ -212,6 +212,11 @@ func TestFeaturesVIMode(t *testing.T) {
 			Shell:         shell.PWSH,
 			ExpectedFeats: shell.VIMode,
 		},
+		{
+			Case:          "fish enables vi mode tracking",
+			Shell:         shell.FISH,
+			ExpectedFeats: shell.VIMode,
+		},
 		{
 			Case:          "bash does not enable vi mode tracking",
 			Shell:         shell.BASH,

src/shell/fish.go

@@ -29,7 +29,9 @@ func (f Features) Fish() Code {
 		return unixUpgrade
 	case Notice:
 		return unixNotice
-	case RPrompt, PoshGit, Azure, LineError, Jobs, Async, KeyHandlers, VIMode:
+	case VIMode:
+		return "_omp_enable_vimode"
+	case RPrompt, PoshGit, Azure, LineError, Jobs, Async, KeyHandlers:
 		fallthrough
 	default:
 		return ""

src/shell/fish_test.go

@@ -19,6 +19,7 @@ set --global _omp_ftcs_marks 1
 set --global _omp_prompt_mark 1
 set --global _omp_cursor_positioning 1
 set --global _omp_enable_streaming 1
+_omp_enable_vimode
 set --global _omp_transient_rprompt 1`
 
 	assert.Equal(t, want, got)

src/shell/scripts/omp.fish

@@ -747,3 +747,38 @@ function omp_repaint_prompt
     set --global _omp_new_prompt 1
     commandline --function repaint
 end
+
+# vi mode tracking — mirror the active fish bind mode into POSH_VI_MODE and
+# re-render the prompt whenever it changes. Enabled through the vimode segment.
+function _omp_enable_vimode
+    # only track when vi (or hybrid) key bindings are active
+    contains -- "$fish_key_bindings" fish_vi_key_bindings fish_hybrid_key_bindings
+    or return
+
+    # translate fish's bind mode into the keymap names the vimode segment expects
+    function _omp_set_vimode
+        switch $fish_bind_mode
+            case default
+                set --global --export POSH_VI_MODE vicmd
+            case replace replace_one
+                set --global --export POSH_VI_MODE replace
+            case insert
+                set --global --export POSH_VI_MODE viins
+            case '*'
+                set --global --export POSH_VI_MODE $fish_bind_mode
+        end
+    end
+
+    # re-render the prompt on every mode change
+    function _omp_render_vimode --on-variable fish_bind_mode
+        _omp_set_vimode
+        omp_repaint_prompt
+    end
+
+    # oh-my-posh renders the mode through the vimode segment, so silence fish's
+    # built-in mode indicator to avoid a duplicate
+    function fish_mode_prompt
+    end
+
+    _omp_set_vimode
+end

website/docs/segments/system/vimode.mdx

@@ -7,12 +7,15 @@ sidebar_label: Vi mode
 ## What
 
 Display the current Vi mode (insert / normal / visual …) in the prompt. Useful
-when using ZSH [keymaps][zsh-vi-mode] via `bindkey -v` or PowerShell PSReadLine
-`-EditMode Vi` so you can tell at a glance which mode you are in.
+when using ZSH [keymaps][zsh-vi-mode] via `bindkey -v`, PowerShell PSReadLine
+`-EditMode Vi`, or fish [vi key bindings][fish-vi-mode] so you can tell at a
+glance which mode you are in.
 
 In ZSH, adding this segment automatically registers a `zle-keymap-select` hook.
 In PowerShell, it registers a PSReadLine [Vi mode change handler][pwsh-vi-mode].
-Both re-render the prompt every time the active mode changes.
+In fish, it registers an [`--on-variable fish_bind_mode`][fish-bind-mode] handler
+(only when vi or hybrid key bindings are active). All of them re-render the
+prompt every time the active mode changes.
 
 :::caution PowerShell cursor indicator
 
@@ -22,6 +25,15 @@ segment in the prompt instead.
 
 :::
 
+:::caution fish mode prompt
+
+fish support overrides `fish_mode_prompt` with an empty function so fish's
+built-in `[N]` / `[I]` mode indicator does not show up next to the mode
+rendered by this segment. Make sure vi key bindings are enabled (e.g.
+`fish_vi_key_bindings`) for the segment to display.
+
+:::
+
 ## Sample Configuration
 
 import Config from "@site/src/components/Config.js";
@@ -51,9 +63,11 @@ import Config from "@site/src/components/Config.js";
 | Name      |   Type   | Description                                                                                                                                    |
 | --------- | :------: | ---------------------------------------------------------------------------------------------------------------------------------------------- |
 | `.Mode`   | `string` | the normalized mode: `insert`, `normal`, `visual`, `viopp`, `replace`, or the raw keymap when unmapped                                         |
-| `.Keymap` | `string` | the raw mode value ([`$KEYMAP`][zsh-keymap] in ZSH, e.g. `main`, `viins`, `vicmd`, `visual`, `viopp`; `viins` or `vicmd` in PowerShell PSReadLine) |
+| `.Keymap` | `string` | the raw mode value ([`$KEYMAP`][zsh-keymap] in ZSH, e.g. `main`, `viins`, `vicmd`, `visual`, `viopp`; `viins` or `vicmd` in PowerShell PSReadLine; `viins`, `vicmd`, `visual` or `replace` in fish) |
 
 [templates]: /docs/configuration/templates
 [zsh-vi-mode]: https://zsh.sourceforge.io/Doc/Release/Zsh-Line-Editor.html#Keymaps
 [zsh-keymap]: https://zsh.sourceforge.io/Doc/Release/Zsh-Line-Editor.html#index-KEYMAP
 [pwsh-vi-mode]: https://learn.microsoft.com/en-us/powershell/module/psreadline/set-psreadlineoption#-vimodechangehandler
+[fish-vi-mode]: https://fishshell.com/docs/current/interactive.html#vi-mode-commands
+[fish-bind-mode]: https://fishshell.com/docs/current/language.html#the-fish-bind-mode-variable