widget: add optional password masking to Editor

This change adds optional password masking to the Editor. To enable
this feature, set the new Mask field to a non-zero rune. Every rune
in the Editor's contents will be replaced by the mask rune in the
visual display, except for newlines. The actual contents of the
editor can still be accessed with Len, Text, and SetText.

Fixes gio#80

Signed-off-by: tainted-bit <sourcehut@taintedbit.com>

widget/editor_test.go

@@ -9,6 +9,7 @@ import (
 	"reflect"
 	"testing"
 	"testing/quick"
+	"unicode"
 
 	"gioui.org/f32"
 	"gioui.org/font/gofont"
@@ -43,6 +44,28 @@ func TestEditor(t *testing.T) {
 	assertCaret(t, e, 1, 4, len("æbc\naøå•"))
 	e.Move(+1)
 	assertCaret(t, e, 1, 4, len("æbc\naøå•"))
+
+	// Ensure that password masking does not affect caret behavior
+	e.Move(-3)
+	assertCaret(t, e, 1, 1, len("æbc\na"))
+	e.Mask = '*'
+	e.Layout(gtx, cache, font, fontSize)
+	assertCaret(t, e, 1, 1, len("æbc\na"))
+	e.Move(-3)
+	assertCaret(t, e, 0, 2, len("æb"))
+	e.Mask = '\U0001F92B'
+	e.Layout(gtx, cache, font, fontSize)
+	e.moveEnd()
+	assertCaret(t, e, 0, 3, len("æbc"))
+
+	// When a password mask is applied, it should replace all visible glyphs
+	for i, line := range e.lines {
+		for j, glyph := range line.Layout {
+			if glyph.Rune != e.Mask && !unicode.IsSpace(glyph.Rune) {
+				t.Errorf("glyph at (%d, %d) is unmasked rune %d", i, j, glyph.Rune)
+			}
+		}
+	}
 }
 
 // assertCaret asserts that the editor caret is at a particular line