From 8fb49fe5072fdc220c5b122df6cb569d67fd9c30 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Adri=C3=A0=20Arrufat?= <adria.arrufat@gmail.com>
Date: Wed, 23 Sep 2026 13:42:36 +0200
Subject: [PATCH] Match Chrome's root scroll size and clamp x too

Only the root scroller's scrollWidth and scrollHeight are floored at the
viewport; html and body keep the document's own height, as in Chrome. The
document's width is body's widest child, and scrollX clamps against it.
---
 src/browser/Frame.zig                   | 11 ++-
 src/browser/tests/element/position.html |  4 +-
 src/browser/tests/mcp_actions.html      |  2 +-
 src/browser/tests/window_scroll.html    | 10 +--
 src/browser/webapi/Element.zig          | 94 ++++++++++++++++++-------
 src/browser/webapi/Window.zig           | 18 +++--
 6 files changed, 96 insertions(+), 43 deletions(-)

diff --git a/src/browser/Frame.zig b/src/browser/Frame.zig
index 8c1d7bcad..d8f8f8d5e 100644
--- a/src/browser/Frame.zig
+++ b/src/browser/Frame.zig
@@ -155,9 +155,14 @@ _focus_fixup_pending: bool = false,
 
 _style_manager: StyleManager,
 
-// Element.documentHeight, valid while the style version and viewport height
-// it was computed for hold.
-_document_height: ?struct { version: usize, viewport: u32, value: f64 } = null,
+// Element.documentExtent, valid while the style version and viewport it was
+// computed for hold.
+_document_extent: ?struct {
+    version: usize,
+    viewport_width: u32,
+    viewport_height: u32,
+    extent: Element.DocumentExtent,
+} = null,
 _script_manager: ScriptManager,
 
 _http_owner: HttpClient.Owner,
diff --git a/src/browser/tests/element/position.html b/src/browser/tests/element/position.html
index eeeace6bc..8b696c269 100644
--- a/src/browser/tests/element/position.html
+++ b/src/browser/tests/element/position.html
@@ -343,11 +343,11 @@
   // per cell, which a document-sized box turned into ~5M walks.
   testing.expectEqual(document.body.clientHeight, document.body.scrollHeight);
   testing.expectEqual(window.innerHeight, document.documentElement.clientHeight);
+  // What the viewport scrolls over: the document, or the viewport itself
   testing.expectEqual(
-    document.documentElement.offsetHeight,
+    Math.max(document.documentElement.offsetHeight, document.documentElement.clientHeight),
     document.documentElement.scrollHeight,
   );
-  testing.expectTrue(document.documentElement.scrollHeight >= document.documentElement.clientHeight);
 }
 </script>
 
diff --git a/src/browser/tests/mcp_actions.html b/src/browser/tests/mcp_actions.html
index 75bda144f..c407bffa6 100644
--- a/src/browser/tests/mcp_actions.html
+++ b/src/browser/tests/mcp_actions.html
@@ -71,6 +71,6 @@
         document.getElementById('rad').addEventListener('change', function() { window.radChanged = true; });
     </script>
     <!-- Room to scroll the window: the document can't scroll past its bottom -->
-    <div style="height: 10000px"></div>
+    <div style="height: 10000px; width: 10000px"></div>
 </body>
 </html>
diff --git a/src/browser/tests/window_scroll.html b/src/browser/tests/window_scroll.html
index dda8a1d51..89c1fa498 100644
--- a/src/browser/tests/window_scroll.html
+++ b/src/browser/tests/window_scroll.html
@@ -2,7 +2,7 @@
 <script src="testing.js"></script>
 
 <!-- Room to scroll: the document can't scroll past its bottom -->
-<div style="height: 10000px"></div>
+<div style="height: 10000px; width: 10000px"></div>
 
 <script id=scrollBy_exists>
   testing.expectEqual('function', typeof window.scrollBy);
@@ -56,9 +56,11 @@
   window.scrollBy(0, 100);
   testing.expectEqual(max, window.scrollY);
 
-  // The document has no honest width, so x isn't bounded
-  window.scrollTo(99999, 0);
-  testing.expectEqual(99999, window.scrollX);
+  // x clamps the same way, against the widest content
+  const maxX = document.documentElement.scrollWidth - window.innerWidth;
+  testing.expectTrue(maxX > 0);
+  window.scrollTo(99999999, 0);
+  testing.expectEqual(maxX, window.scrollX);
 
   // Content added at the bottom extends the limit
   const more = document.createElement('div');
diff --git a/src/browser/webapi/Element.zig b/src/browser/webapi/Element.zig
index 25e37a6d4..91b43fefd 100644
--- a/src/browser/webapi/Element.zig
+++ b/src/browser/webapi/Element.zig
@@ -1500,11 +1500,11 @@ pub fn getElementAxis(self: *Element, frame: *Frame, comptime axis: Axis) Axis.S
         }
     }
 
-    // Root containers span the document, see documentHeight.
+    // Root containers span the document, see documentExtent.
     if (root) {
         return .{ .value = switch (axis) {
             .width => 1920.0,
-            .height => if (self.ownerFrame(frame)) |owner| documentHeight(owner) else 0.0,
+            .height => if (self.ownerFrame(frame)) |owner| documentExtent(owner).height else 0.0,
         } };
     }
 
@@ -1539,6 +1539,13 @@ fn clientAxis(self: *Element, frame: *Frame, comptime axis: Axis) f64 {
     return self.viewportAxis(frame, axis) orelse self.boxAxis(frame, axis);
 }
 
+/// documentScrollSize, when self is the root scroller (see viewportAxis).
+fn rootScrollSize(self: *Element, frame: *Frame, comptime axis: Axis) ?f64 {
+    _ = self.viewportAxis(frame, axis) orelse return null;
+    const owner = self.ownerFrame(frame) orelse return null;
+    return @field(documentScrollSize(owner), @tagName(axis));
+}
+
 fn viewportAxis(self: *Element, frame: *Frame, comptime axis: Axis) ?f64 {
     const tag = self.getTag();
     if (tag != .html and tag != .body) {
@@ -1679,9 +1686,10 @@ pub fn getScrollHeight(self: *Element, frame: *Frame) f64 {
     const height = self.getElementAxis(frame, .height).value;
 
     const tag = self.getTag();
-    // The root containers already span the document.
+    // The root containers already span the document. The one scrolling the
+    // viewport reports what the viewport scrolls over.
     if (tag == .html or tag == .body) {
-        return height;
+        return self.rootScrollSize(frame, .height) orelse height;
     }
 
     return @max(height, self.contentAxis(frame, .height));
@@ -1695,11 +1703,12 @@ pub fn getScrollWidth(self: *Element, frame: *Frame) f64 {
     const width = self.getElementAxis(frame, .width).value;
 
     const tag = self.getTag();
-    // The root containers span the document (see getElementAxis), and
-    // their children don't lie side by side on one row: stacking them
-    // would inflate a value sites read to detect page overflow.
+    // The root containers keep their box: their children don't lie side by
+    // side on one row, and stacking them would inflate a value sites read to
+    // detect page overflow. The one scrolling the viewport reports what the
+    // viewport scrolls over.
     if (tag == .html or tag == .body) {
-        return width;
+        return self.rootScrollSize(frame, .width) orelse width;
     }
 
     return @max(width, self.contentAxis(frame, .width));
@@ -1912,35 +1921,68 @@ fn calculateDocumentPosition(node: *Node) f64 {
     return position * 5.0; // 5px per node
 }
 
-/// The document's height, which html and body span and the viewport scrolls
-/// over. Tall enough for every synthetic position (calculateDocumentPosition's
-/// 5px per node), for body's children stacked on top of each other, and never
-/// shorter than the viewport, like the root's scrollHeight.
-pub fn documentHeight(frame: *Frame) f64 {
+pub const DocumentExtent = struct { width: f64, height: f64 };
+
+/// The document's extent. Its height, which html and body span, fits every
+/// synthetic position (calculateDocumentPosition's 5px per node) and body's
+/// children stacked on top of each other. Its width is body's widest child:
+/// block children don't lie side by side on one row. An inline size on body
+/// stretches both.
+pub fn documentExtent(frame: *Frame) DocumentExtent {
     const version = frame.page.style_version;
-    const viewport = frame.page.getViewport().height;
-    if (frame._document_height) |cached| {
-        if (cached.version == version and cached.viewport == viewport) {
-            return cached.value;
+    const viewport = frame.page.getViewport();
+    if (frame._document_extent) |cached| {
+        if (cached.version == version and cached.viewport_width == viewport.width and cached.viewport_height == viewport.height) {
+            return cached.extent;
         }
     }
 
     const doc = frame.document;
-    var height = @max(@as(f64, @floatFromInt(viewport)), countSubtreeNodes(doc.asNode()) * 5.0);
-    if (doc.getDocumentElement()) |root| {
-        var child = root.asNode().firstChild();
+    var extent: DocumentExtent = .{ .width = 0, .height = countSubtreeNodes(doc.asNode()) * 5.0 };
+    if (findBody(doc)) |body| {
+        const style_manager = &frame._style_manager;
+        extent.height = @max(extent.height, body.contentAxis(frame, .height), style_manager.inlineSize(body, .height) orelse 0);
+        extent.width = style_manager.inlineSize(body, .width) orelse 0;
+        var child = body.asNode().firstChild();
         while (child) |node| : (child = node.nextSibling()) {
-            if (node.is(Html.Body)) |body| {
-                height = @max(height, body.asElement().contentAxis(frame, .height));
-                break;
+            const el = node.is(Element) orelse continue;
+            if (!style_manager.hasDisplayNone(el)) {
+                extent.width = @max(extent.width, el.getElementAxis(frame, .width).value);
             }
         }
     }
 
     // Whole pixels, like the scroll offsets clamped against it
-    height = @ceil(height);
-    frame._document_height = .{ .version = version, .viewport = viewport, .value = height };
-    return height;
+    extent = .{ .width = @ceil(extent.width), .height = @ceil(extent.height) };
+    frame._document_extent = .{
+        .version = version,
+        .viewport_width = viewport.width,
+        .viewport_height = viewport.height,
+        .extent = extent,
+    };
+    return extent;
+}
+
+/// What the viewport scrolls over: the document, but never less than the
+/// viewport itself, like the root scroller's scrollWidth and scrollHeight.
+pub fn documentScrollSize(frame: *Frame) DocumentExtent {
+    const extent = documentExtent(frame);
+    const viewport = frame.page.getViewport();
+    return .{
+        .width = @max(extent.width, @as(f64, @floatFromInt(viewport.width))),
+        .height = @max(extent.height, @as(f64, @floatFromInt(viewport.height))),
+    };
+}
+
+fn findBody(doc: *Node.Document) ?*Element {
+    const root = doc.getDocumentElement() orelse return null;
+    var child = root.asNode().firstChild();
+    while (child) |node| : (child = node.nextSibling()) {
+        if (node.is(Html.Body)) |body| {
+            return body.asElement();
+        }
+    }
+    return null;
 }
 
 // Counts total nodes in a subtree (node + all descendants)
diff --git a/src/browser/webapi/Window.zig b/src/browser/webapi/Window.zig
index bffa6c04e..bede248ac 100644
--- a/src/browser/webapi/Window.zig
+++ b/src/browser/webapi/Window.zig
@@ -951,13 +951,12 @@ fn getDevicePixelRatio(_: *const Window, frame: *Frame) f32 {
 
 pub fn scrollTo(self: *Window, opts: Element.ScrollToOpts, y: ?i32, frame: *Frame) !void {
     const o = opts.offsets(y);
-    const new_x: u32 = if (o.left) |left| @intCast(@max(0, left)) else self._scroll_pos.x;
-    var new_y: u32 = if (o.top) |top| @intCast(@max(0, top)) else self._scroll_pos.y;
-
-    // The document can't scroll past its bottom. It has no honest width
-    // (children don't lie side by side), so x stays unbounded.
-    const max_y = Element.documentHeight(self._frame) - @as(f64, @floatFromInt(self.getInnerHeight(self._frame)));
-    new_y = @min(new_y, @as(u32, @intFromFloat(@max(0, @min(max_y, std.math.maxInt(u32))))));
+    // The viewport can't scroll past the document's edges
+    const size = Element.documentScrollSize(self._frame);
+    const max_x = scrollLimit(size.width, self.getInnerWidth(self._frame));
+    const max_y = scrollLimit(size.height, self.getInnerHeight(self._frame));
+    const new_x: u32 = if (o.left) |left| @min(@as(u32, @intCast(@max(0, left))), max_x) else self._scroll_pos.x;
+    const new_y: u32 = if (o.top) |top| @min(@as(u32, @intCast(@max(0, top))), max_y) else self._scroll_pos.y;
 
     if (new_x == self._scroll_pos.x and new_y == self._scroll_pos.y) {
         return;
@@ -1019,6 +1018,11 @@ pub fn scrollTo(self: *Window, opts: Element.ScrollToOpts, y: ?i32, frame: *Fram
     );
 }
 
+fn scrollLimit(size: f64, visible: u32) u32 {
+    const limit = size - @as(f64, @floatFromInt(visible));
+    return @intFromFloat(std.math.clamp(limit, 0, std.math.maxInt(u32)));
+}
+
 pub fn scrollBy(self: *Window, opts: Element.ScrollToOpts, y: ?i32, frame: *Frame) !void {
     const o = opts.offsets(y);
     // The viewport has no honest extent, so a stored offset can sit above