Don't share a live view's buffer when copying its content

moveMainContextToTop copies the current top view's content into the view
it's promoting, to avoid a flicker. The source can be a main view with a
live streaming task (e.g. resolving a conflict promotes the merge-conflicts
view over a main view that's mid-diff), and CopyContent both read and
published that source's buffer unsafely:

  - it read the source's lines/viewLines while locking only the
    destination, racing the task's concurrent Write; and
  - it aliased the source's row slices into the destination, so the
    source's ongoing appends (growslice reading the shared array) and
    refreshViewLinesIfNeeded's in-place wrapping-cache writes (&lines[i])
    kept racing this view's rendering after the copy.

Lock the source for the read, and shallow-clone the row slices so the
destination gets its own arrays. The per-row cell data is immutable once
written, so it stays shared -- the clone cost is proportional to the
number of rows, not their contents.

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

pkg/gocui/view.go

@@ -7,6 +7,7 @@ package gocui
 import (
 	"fmt"
 	"io"
+	"slices"
 	"strings"
 	"sync"
 	"unicode"
@@ -1146,10 +1147,25 @@ func (v *View) CopyContent(from *View) {
 	v.writeMutex.Lock()
 	defer v.writeMutex.Unlock()
 
+	// A background task may be streaming output into the source view's buffer
+	// via Write, so read it under its own lock. The source is always a
+	// different view than the destination (see the sole caller,
+	// moveMainContextToTop), and no other code holds two view write locks at
+	// once, so this can't deadlock.
+	from.writeMutex.Lock()
+	defer from.writeMutex.Unlock()
+
 	v.clear()
 
-	v.lines = from.lines
-	v.viewLines = from.viewLines
+	// Clone the row slices rather than sharing them: the source view stays
+	// live (its streaming task keeps appending rows, and refreshViewLinesIfNeeded
+	// fills each row's wrapping cache in place via &lines[i]), so sharing the
+	// backing arrays would race those writes against this view's own rendering.
+	// This is a shallow clone -- the per-row cell data is immutable once written
+	// and stays shared, so the cost is proportional to the number of rows, not
+	// their contents.
+	v.lines = slices.Clone(from.lines)
+	v.viewLines = slices.Clone(from.viewLines)
 	v.ox = from.ox
 	v.oy = from.oy
 	v.cx = from.cx