Add IsUIThread and OnUIThreadAndWait to gocui

The next commits move refresh workers to read UI-thread-owned state (the
model, contexts, selection) on the UI thread rather than off it. Two
primitives support that:

- OnUIThreadAndWait runs a function on the main event loop and blocks the
  caller until it has run, so a worker can read that state without racing.
  OnUIThreadAndWaitBackground is the same for background routines, whose
  work must not count towards the program being busy.
- IsUIThread reports whether the caller is on the main event loop, for a
  debug-only assertion that a refresh was issued from the thread it claims.
  It records the main loop's goroutine id in MainLoop and compares via
  goid, so it's promoted from an indirect to a direct dependency.

goid is used only by that debug assertion, never to drive production
control flow.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

go.mod

@@ -25,6 +25,7 @@ require (
 	github.com/lucasb-eyer/go-colorful v1.4.0
 	github.com/mgutz/str v1.2.0
 	github.com/mitchellh/go-ps v1.0.0
+	github.com/petermattis/goid v0.0.0-20250813065127-a731cc31b4fe
 	github.com/rivo/uniseg v0.4.7
 	github.com/sahilm/fuzzy v0.1.3
 	github.com/samber/lo v1.53.0
@@ -62,7 +63,6 @@ require (
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/onsi/ginkgo v1.10.3 // indirect
 	github.com/onsi/gomega v1.34.1 // indirect
-	github.com/petermattis/goid v0.0.0-20250813065127-a731cc31b4fe // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/wk8/go-ordered-map/v2 v2.1.8 // indirect
 	golang.org/x/mod v0.35.0 // indirect

pkg/gocui/gui.go

@@ -9,11 +9,13 @@ import (
 	"runtime"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/gdamore/tcell/v3"
 	"github.com/go-errors/errors"
 	"github.com/jesseduffield/generics/set"
+	"github.com/petermattis/goid"
 	"github.com/rivo/uniseg"
 	"github.com/samber/lo"
 )
@@ -200,6 +202,11 @@ type Gui struct {
 	currentTask Task
 
 	lastHoverView *View
+
+	// uiThreadID is the goroutine id of the main event loop, recorded when
+	// MainLoop starts. IsUIThread compares against it. Written once, read from
+	// worker goroutines, so it's atomic.
+	uiThreadID atomic.Int64
 }
 
 type NewGuiOpts struct {
@@ -684,6 +691,46 @@ func (g *Gui) updateContentOnly(f func(*Gui) error, background bool) {
 	g.userEvents <- userEvent{f: f, task: task, contentOnly: true}
 }
 
+// IsUIThread reports whether the caller is running on the main event-loop
+// goroutine (the one running MainLoop). It calls goid.Get, so use it only for
+// debug assertions, not to drive production control flow.
+func (g *Gui) IsUIThread() bool {
+	return goid.Get() == g.uiThreadID.Load()
+}
+
+// OnUIThreadAndWait runs f on the main event-loop goroutine and blocks the
+// caller until f has run, returning f's error. Use it to read UI-thread-owned
+// state (the model, contexts) from a worker without racing the UI thread.
+//
+// It must be called from a worker goroutine, never from the UI thread itself:
+// the UI thread would block waiting for a callback only it can run, which
+// deadlocks. Callers arrange this by construction (see the refresh helper's
+// RefreshFromWorker); a debug-only assertion there guards against getting it
+// wrong.
+func (g *Gui) OnUIThreadAndWait(f func() error) error {
+	return g.onUIThreadAndWait(f, false)
+}
+
+// Like OnUIThreadAndWait, but the enqueued work belongs to a background routine,
+// so it doesn't count towards the program being busy (see UpdateBackground).
+func (g *Gui) OnUIThreadAndWaitBackground(f func() error) error {
+	return g.onUIThreadAndWait(f, true)
+}
+
+func (g *Gui) onUIThreadAndWait(f func() error, background bool) error {
+	enqueue := g.Update
+	if background {
+		enqueue = g.UpdateBackground
+	}
+
+	result := make(chan error, 1)
+	enqueue(func(*Gui) error {
+		result <- f()
+		return nil
+	})
+	return <-result
+}
+
 // Calls a function in a goroutine. Handles panics gracefully and tracks
 // number of background tasks.
 // Always use this when you want to spawn a goroutine and you want lazygit to
@@ -766,6 +813,8 @@ func (g *Gui) SetManagerFunc(manager func(*Gui) error) {
 // MainLoop runs the main loop until an error is returned. A successful
 // finish should return ErrQuit.
 func (g *Gui) MainLoop() error {
+	g.uiThreadID.Store(goid.Get())
+
 	go func() {
 		for {
 			select {