widget: fix label vertical glyph padding logic

We previously were not handling glyphs that extended vertically beyond the
ascent/descent declared by their font. This is done rarely with text fonts,
but is apparently common among symbol and emoji fonts.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/label.go

@@ -150,11 +150,26 @@ func (it *textIterator) processGlyph(g text.Glyph, ok bool) (_ text.Glyph, visib
 	// Compute the maximum extent to which glyphs overhang on the horizontal
 	// axis.
 	if d := g.Bounds.Min.X.Floor(); d < it.padding.Min.X {
+		// If the distance between the dot and the left edge of this glyph is
+		// less than the current padding, increase the left padding.
 		it.padding.Min.X = d
 	}
 	if d := (g.Bounds.Max.X - g.Advance).Ceil(); d > it.padding.Max.X {
+		// If the distance between the dot and the right edge of this glyph
+		// minus the logical advance of this glyph is greater than the current
+		// padding, increase the right padding.
 		it.padding.Max.X = d
 	}
+	if d := (g.Bounds.Min.Y + g.Ascent).Floor(); d < it.padding.Min.Y {
+		// If the distance between the dot and the top of this glyph is greater
+		// than the ascent of the glyph, increase the top padding.
+		it.padding.Min.Y = d
+	}
+	if d := (g.Bounds.Max.Y - g.Descent).Ceil(); d > it.padding.Max.Y {
+		// If the distance between the dot and the bottom of this glyph is greater
+		// than the descent of the glyph, increase the bottom padding.
+		it.padding.Max.Y = d
+	}
 	logicalBounds := image.Rectangle{
 		Min: image.Pt(g.X.Floor(), int(g.Y)-g.Ascent.Ceil()),
 		Max: image.Pt((g.X + g.Advance).Ceil(), int(g.Y)+g.Descent.Ceil()),

widget/label_test.go

@@ -166,3 +166,67 @@ func TestGlyphIterator(t *testing.T) {
 		})
 	}
 }
+
+// TestGlyphIteratorPadding ensures that the glyph iterator computes correct padding
+// around glyphs with unusual bounding boxes.
+func TestGlyphIteratorPadding(t *testing.T) {
+	type testcase struct {
+		name             string
+		glyph            text.Glyph
+		viewport         image.Rectangle
+		expectedDims     image.Rectangle
+		expectedPadding  image.Rectangle
+		expectedBaseline int
+	}
+	for _, tc := range []testcase{
+		{
+			name: "simple",
+			glyph: text.Glyph{
+				X:       0,
+				Y:       50,
+				Advance: fixed.I(50),
+				Ascent:  fixed.I(50),
+				Descent: fixed.I(50),
+				Bounds: fixed.Rectangle26_6{
+					Min: fixed.Point26_6{
+						X: fixed.I(-5),
+						Y: fixed.I(-56),
+					},
+					Max: fixed.Point26_6{
+						X: fixed.I(57),
+						Y: fixed.I(58),
+					},
+				},
+			},
+			viewport: image.Rectangle{Max: image.Pt(math.MaxInt, math.MaxInt)},
+			expectedDims: image.Rectangle{
+				Max: image.Point{X: 50, Y: 100},
+			},
+			expectedBaseline: 50,
+			expectedPadding: image.Rectangle{
+				Min: image.Point{
+					X: -5,
+					Y: -6,
+				},
+				Max: image.Point{
+					X: 7,
+					Y: 8,
+				},
+			},
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			it := textIterator{viewport: tc.viewport}
+			it.processGlyph(tc.glyph, true)
+			if it.bounds != tc.expectedDims {
+				t.Errorf("expected bounds %#+v, got %#+v", tc.expectedDims, it.bounds)
+			}
+			if it.baseline != tc.expectedBaseline {
+				t.Errorf("expected baseline %d, got %d", tc.expectedBaseline, it.baseline)
+			}
+			if it.padding != tc.expectedPadding {
+				t.Errorf("expected padding %d, got %d", tc.expectedPadding, it.padding)
+			}
+		})
+	}
+}