Bug 1918454 - Prevent divide by zero when inverting effective zoom. r=firefox-style-system-reviewers,layout-reviewers,boris

See comment.

Differential Revision: https://phabricator.services.mozilla.com/D222090

servo/components/style/properties/properties.mako.rs

@@ -2726,7 +2726,10 @@ impl<'a> StyleBuilder<'a> {
     pub fn resolved_specified_zoom(&self) -> computed::Zoom {
         let zoom = self.specified_zoom();
         if zoom.is_document() {
-            self.inherited_effective_zoom().inverted()
+            // If our inherited effective zoom has derived to zero, there's not much we can do.
+            // This value is not exposed to content anyways (it's used for scrollbars and to avoid
+            // zoom affecting canvas).
+            self.inherited_effective_zoom().inverted().unwrap_or(computed::Zoom::ONE)
         } else {
             zoom
         }

servo/components/style/values/computed/box.rs

@@ -365,8 +365,11 @@ impl Zoom {
 
     /// Returns the inverse of our value.
     #[inline]
-    pub fn inverted(&self) -> Self {
-        Self(Self::ONE.0 / self.0)
+    pub fn inverted(&self) -> Option<Self> {
+        if self.0.value == 0 {
+            return None;
+        }
+        Some(Self(Self::ONE.0 / self.0))
     }
 
     /// Returns the value as a float.

testing/web-platform/tests/css/css-viewport/zoom/textarea-very-small-zoom-crash.html

@@ -0,0 +1,6 @@
+<style>
+*:nth-of-type(1) {
+  zoom: 5%;
+}
+</style>
+<textarea>