LibWeb: Make horizontal scrollbar start from the left edge of viewport

Horizontal scrollbar has to leave space at right edge for the vertical
scrollbar to fully extend from top-to-bottom edge of viewport. Before,
this was done by just moving it leftward beyond the edge of viewport.

Now, it gets scaled down appropriately to fit between left edge of
viewport & vertical scrollbar without clipping.

Userland/Libraries/LibWeb/Painting/PaintableBox.cpp

@@ -280,25 +280,28 @@ Optional<PaintableBox::ScrollbarData> PaintableBox::compute_scrollbar_data(Scrol
         return {};
     }
 
+    bool is_horizontal = direction == ScrollDirection::Horizontal;
+
     auto padding_rect = absolute_padding_box_rect();
     auto scrollable_overflow_rect = this->scrollable_overflow_rect().value();
-    auto scroll_overflow_size = direction == ScrollDirection::Horizontal ? scrollable_overflow_rect.width() : scrollable_overflow_rect.height();
-    auto scrollport_size = direction == ScrollDirection::Horizontal ? padding_rect.width() : padding_rect.height();
+    auto scroll_overflow_size = is_horizontal ? scrollable_overflow_rect.width() : scrollable_overflow_rect.height();
+    auto scrollport_size = is_horizontal ? padding_rect.width() : padding_rect.height();
     if (scroll_overflow_size == 0)
         return {};
 
-    auto min_thumb_length = min(scrollport_size, 24);
-    auto thumb_length = max(scrollport_size * (scrollport_size / scroll_overflow_size), min_thumb_length);
+    auto scrollbar_rect_length = is_horizontal ? scrollport_size - scrollbar_thumb_thickness : scrollport_size;
+
+    auto min_thumb_length = min(scrollbar_rect_length, 24);
+    auto thumb_length = max(scrollbar_rect_length * (scrollport_size / scroll_overflow_size), min_thumb_length);
 
     CSSPixelFraction scroll_size = 0;
     if (scroll_overflow_size > scrollport_size)
-        scroll_size = (scrollport_size - thumb_length) / (scroll_overflow_size - scrollport_size);
+        scroll_size = (scrollbar_rect_length - thumb_length) / (scroll_overflow_size - scrollport_size);
     CSSPixelRect rect;
-    if (direction == ScrollDirection::Vertical) {
+    if (is_horizontal)
+        rect = { padding_rect.left(), padding_rect.bottom() - scrollbar_thumb_thickness, thumb_length, scrollbar_thumb_thickness };
+    else
         rect = { padding_rect.right() - scrollbar_thumb_thickness, padding_rect.top(), scrollbar_thumb_thickness, thumb_length };
-    } else {
-        rect = { padding_rect.left() - scrollbar_thumb_thickness, padding_rect.bottom() - scrollbar_thumb_thickness, thumb_length, scrollbar_thumb_thickness };
-    }
 
     return PaintableBox::ScrollbarData { rect, scroll_size };
 }