M12: an XFA button without a height is as tall as its caption

A button with no height was sized like a text field in its field font,
with text-field padding, so DA Form 31's Instructions button (10 pt
field font, 8 pt caption) came out about 17 pt tall and over the form
below it. It is now as tall as its caption, never below its minimum.

crates/xfa/src/layout.rs

@@ -1060,6 +1060,18 @@ impl Layouter<'_> {
         let lines = if f.multiline { 3.0 } else { 1.0 };
         let ui = match f.ui {
             Ui::CheckButton => f.check_size.unwrap_or(10.0).max(text::line_height(font.size)),
+            // A button shows its caption: as tall as the caption's text in its own font.
+            Ui::Button if f.caption.as_ref().is_some_and(|c| !c.value.is_empty()) => {
+                let cap = f.caption.as_ref().map_or(0.0, |c| {
+                    let rich = match &c.value {
+                        Value::Rich(r) => r.clone(),
+                        other => text::plain(&other.plain().unwrap_or_default()),
+                    };
+                    let w = (inner_w - f.ui_margin.horizontal()).max(1.0);
+                    text::measure_height(&rich, w, &ctx.para.apply(&c.para), &Face::of(&ctx.font.apply(&c.font)), &|id| self.embed(id))
+                });
+                cap + f.ui_margin.vertical()
+            }
             _ => (text::line_height(font.size) * lines).max(self.grown_text_height(f, inner_w, ctx)) + 4.0 + f.ui_margin.vertical(),
         };
         match self.caption_reserve(f, inner_w, ctx) {

crates/xfa/src/tests.rs

@@ -1252,6 +1252,20 @@ fn a_button_without_a_width_is_as_wide_as_its_caption() {
     assert!((button.rect.w - 200.0).abs() < 0.05, "{}", button.rect.w);
 }
 
+#[test]
+fn a_button_without_a_height_is_as_tall_as_its_caption() {
+    // A 10pt field font, an 8pt caption: the button is as tall as its caption's line (no
+    // text-field padding), and never below its minimum height.
+    let button = |min_h: &str| {
+        let items = positioned(&format!(
+            r#"<field name="help" x="1in" y="0" w="100pt" minH="{min_h}"><ui><button/></ui><font typeface="Times New Roman" size="10pt"/><caption><value><text>Help</text></value><font typeface="Arial" size="8pt"/></caption></field>"#
+        ));
+        items.iter().find_map(|i| if let Item::Widget(w) = i { Some(w.rect.h) } else { None }).expect("the button")
+    };
+    assert!((button("2pt") - text::line_height(8.0)).abs() < 0.05, "{}", button("2pt"));
+    assert!((button("10.5pt") - 10.5).abs() < 0.05, "{}", button("10.5pt"));
+}
+
 #[test]
 fn page_area_without_occur_serves_every_body_page() {
     let section = |n: u32| {