editor: add HandleEnd window-base regression test

Cover the base-offset conversion in HandleEnd (the path that slices the full
file content by the end-of-line offset), mirroring the tap-to-position
regression test.

internal/editor/cursor_test.go

@@ -154,6 +154,39 @@ func TestTapToPosition_WindowBaseOffset(t *testing.T) {
 	}
 }
 
+// TestHandleEnd_WindowBaseOffset is a regression test for the window-relative
+// GlyphLayout bug in HandleEnd. The layout covers only the visible window
+// (base = IMEWindowStartByte), so its ByteOffsets are window-relative.
+// HandleEnd converts the end-of-line position to an absolute file offset
+// (base + window offset) before using it to index the full file content.
+// Before the fix it used the window-relative offset, landing on the previous
+// line's newline.
+func TestHandleEnd_WindowBaseOffset(t *testing.T) {
+	lh := float64(EditorLineHeight())
+	// File: line0 "AAAAAAAA\n" (bytes 0-8), line1 "BBBBBBBB\n" (bytes 9-17).
+	// Window starts at byte 9 (line 1). Window-relative offsets: B=0..7, \n=8.
+	gl := ui.GlyphLayout{LineHeight: EditorLineHeight()}
+	for i := 0; i < 9; i++ {
+		gl.ByteOffsets = append(gl.ByteOffsets, i)
+		gl.X = append(gl.X, ui.Dp(10+10*i))
+		gl.Y = append(gl.Y, ui.Dp(lh)) // single visual line
+		gl.Advance = append(gl.Advance, 10)
+	}
+	TheState = NewState()
+	TheState.Editor.Buffer = "AAAAAAAA\nBBBBBBBB\n"
+	TheState.Editor.GlyphLayout = gl
+	TheState.Editor.IMEWindowStartByte = 9
+	TheState.Editor.CursorPosition = 12 // line 1, 4th B (absolute)
+
+	HandleEnd()
+
+	// The end of line 1 is its newline at absolute byte 17. Before the fix the
+	// window-relative offset (8) indexed the previous line's newline (byte 8).
+	if TheState.Editor.CursorPosition != 17 {
+		t.Errorf("HandleEnd cursor = %d, want 17 (newline ending line 1); window-relative offset bug?", TheState.Editor.CursorPosition)
+	}
+}
+
 // TestTapToPosition_LargeFileScrolled is the end-to-end regression: with a
 // window-relative GlyphLayout and a deep scroll, a tap a few lines below the top
 // of the window must land on that window line, NOT be clamped to the last window