render: CSS width/height are now children-content aware

This takes https://github.com/lightpanda-io/browser/pull/3137 and applies is to
the width/height CSS values (e.g. CSSStyleCSSStyle.getPropertyValue("width")) to
provide a consistent width/height view of an element.

Fixes slow rendering of https://inchurch.com.br

src/browser/tests/element/position.html

@@ -168,6 +168,38 @@
 }
 </script>
 
+<script id="computedWidthMarqueeLoop">
+{
+  // The getComputedStyle variant of the marquee idiom (inchurch.com.br's
+  // dynamic-marquee-widget via jQuery .width()): the computed value must
+  // carry the same content fallback as clientWidth or the loop never
+  // terminates.
+  const container = document.createElement('div');
+  const wrap = document.createElement('div');
+  const item = document.createElement('span');
+  wrap.appendChild(item);
+  container.appendChild(wrap);
+  document.body.appendChild(container);
+
+  const width = (el) => parseFloat(getComputedStyle(el).width);
+  const containerWidth = width(container);
+  let guard = 0;
+  while (width(wrap) < containerWidth * 2 && guard < 50) {
+    wrap.appendChild(item.cloneNode(true));
+    guard++;
+  }
+  testing.expectTrue(guard < 50);
+
+  // An explicit inline size still wins over the content sum.
+  const pinned = document.createElement('div');
+  pinned.style.width = '40px';
+  pinned.appendChild(document.createElement('span'));
+  pinned.appendChild(document.createElement('span'));
+  document.body.appendChild(pinned);
+  testing.expectEqual('40px', getComputedStyle(pinned).width);
+}
+</script>
+
 <script id="scrollHeightFromContent">
 {
   // An empty element has no content to overflow its box.

src/browser/webapi/Element.zig

@@ -1364,7 +1364,11 @@ pub fn getElementDimensions(self: *Element, frame: *Frame) Dimensions {
 // to contentWidth/contentHeight
 pub fn getClientWidth(self: *Element, frame: *Frame) f64 {
     var visibility_cache: VisibilityCache = .{};
-    if (!self.checkVisibilityCached(&visibility_cache, frame)) {
+    return self.getClientWidthWithCache(frame, &visibility_cache);
+}
+
+pub fn getClientWidthWithCache(self: *Element, frame: *Frame, visibility_cache: *VisibilityCache) f64 {
+    if (!self.checkVisibilityCached(visibility_cache, frame)) {
         return 0.0;
     }
 
@@ -1375,12 +1379,16 @@ pub fn getClientWidth(self: *Element, frame: *Frame) f64 {
         return dims.width;
     }
 
-    return @max(dims.width, self.contentWidth(frame, &visibility_cache));
+    return @max(dims.width, self.contentWidth(frame, visibility_cache));
 }
 
 pub fn getClientHeight(self: *Element, frame: *Frame) f64 {
     var visibility_cache: VisibilityCache = .{};
-    if (!self.checkVisibilityCached(&visibility_cache, frame)) {
+    return self.getClientHeightWithCache(frame, &visibility_cache);
+}
+
+pub fn getClientHeightWithCache(self: *Element, frame: *Frame, visibility_cache: *VisibilityCache) f64 {
+    if (!self.checkVisibilityCached(visibility_cache, frame)) {
         return 0.0;
     }
 
@@ -1391,7 +1399,7 @@ pub fn getClientHeight(self: *Element, frame: *Frame) f64 {
         return dims.height;
     }
 
-    return @max(dims.height, self.contentHeight(frame, &visibility_cache));
+    return @max(dims.height, self.contentHeight(frame, visibility_cache));
 }
 
 pub fn getBoundingClientRect(self: *Element, frame: *Frame) !*DOMRect {

src/browser/webapi/css/CSSStyleDeclaration.zig

@@ -97,9 +97,11 @@ pub fn getPropertyValue(self: *const CSSStyleDeclaration, property_name: []const
                 }
 
                 // Computed width/height must agree with the synthetic layout
-                // metrics (offsetWidth/getBoundingClientRect). Returning ""
-                // makes measurement code see contradictory sizes — jQuery's
-                // "shrink text until it fits" loops then never terminate.
+                // metrics. Returning "" makes measurement code see
+                // contradictory sizes — jQuery's "shrink text until it fits"
+                // loops then never terminate. jQuery's .width() reads this
+                // value, so it must also carry clientWidth's content fallback
+                // or append-until-wide marquee loops never terminate.
                 if (wrapped.eql(comptime .wrap("width"))) {
                     return resolvedDimension(element, .width, frame);
                 }
@@ -115,13 +117,13 @@ pub fn getPropertyValue(self: *const CSSStyleDeclaration, property_name: []const
 }
 
 fn resolvedDimension(element: *Element, dimension: enum { width, height }, frame: *Frame) []const u8 {
-    if (!element.checkVisibilityCached(null, frame)) {
+    var visibility_cache: Element.VisibilityCache = .{};
+    if (!element.checkVisibilityCached(&visibility_cache, frame)) {
         return "auto";
     }
-    const dims = element.getElementDimensions(frame);
     const value = switch (dimension) {
-        .width => dims.width,
-        .height => dims.height,
+        .width => element.getClientWidthWithCache(frame, &visibility_cache),
+        .height => element.getClientHeightWithCache(frame, &visibility_cache),
     };
     return std.fmt.allocPrint(frame.local_arena, "{d}px", .{value}) catch "auto";
 }