Enhance separator logic to handle most use cases.

In https://chromium-review.googlesource.com/c/chromium/src/+/1880531, we
fixed the case where a separator would not properly fill its bounds.
However that broke functionality where a spacer was being used as e.g. a
vertical separator in a text flow with insets on either side (see
attached bug).

This solution does both:
 - when there is no inset on a particular side of the separator, the
   separator is guaranteed to draw flush with the edge of the view
 - when there is an inset, the inset is guaranteed to be preserved,
   with the old behavior of the separator being adjusted to draw only on
   whole pixels in DPI-scaled environments (minimum of 1px in either
   dimension)


Fixes: 1019503
Bug: 1019503
Change-Id: I97dd487ba2374000dcef76e43665ef44eb93acd8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894253
Commit-Queue: Dana Fried <dfried@chromium.org>
Auto-Submit: Dana Fried <dfried@chromium.org>
Reviewed-by: Collin Baker <collinbaker@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711842}

ui/views/controls/separator.cc

@@ -7,6 +7,7 @@
 #include "ui/accessibility/ax_enums.mojom.h"
 #include "ui/accessibility/ax_node_data.h"
 #include "ui/gfx/canvas.h"
+#include "ui/gfx/scoped_canvas.h"
 #include "ui/native_theme/native_theme.h"
 
 namespace views {
@@ -56,8 +57,40 @@ void Separator::OnPaint(gfx::Canvas* canvas) {
                             ? *overridden_color_
                             : GetNativeTheme()->GetSystemColor(
                                   ui::NativeTheme::kColorId_SeparatorColor);
-  canvas->DrawColor(color);
+  // Paint background and border, if any.
   View::OnPaint(canvas);
+
+  gfx::ScopedCanvas scoped_canvas(canvas);
+  const gfx::Insets insets = GetInsets();
+  const gfx::Rect contents_bounds = GetContentsBounds();
+  const float dsf = canvas->UndoDeviceScaleFactor();
+
+  // This is a hybrid of gfx::ScaleToEnclosedRect() and
+  // gfx::ScaleToEnclosingRect() based on whether there are nonzero insets on
+  // any particular side of the separator. If there is no inset, the separator
+  // will paint all the way out to the edge of the view. If there is an inset,
+  // the extent of the separator will rounded inward so that it paints only
+  // full pixels, providing a sharper appearance and preserving the inset.
+  //
+  // This allows separators that entirely fill their area to do so, and those
+  // intended as spacers in a larger flow to do so as well. See
+  // crbug.com/1016760 and crbug.com/1019503 for examples of why we need to
+  // handle both cases.
+  const int x = insets.left() == 0 ? std::floor(contents_bounds.x() * dsf)
+                                   : std::ceil(contents_bounds.x() * dsf);
+  const int y = insets.top() == 0 ? std::floor(contents_bounds.y() * dsf)
+                                  : std::ceil(contents_bounds.y() * dsf);
+  const int r = insets.right() == 0 ? std::ceil(contents_bounds.right() * dsf)
+                                    : std::floor(contents_bounds.right() * dsf);
+  const int b = insets.bottom() == 0
+                    ? std::ceil(contents_bounds.bottom() * dsf)
+                    : std::floor(contents_bounds.bottom() * dsf);
+
+  // Minimum separator size is 1 px.
+  const int w = std::max(1, r - x);
+  const int h = std::max(1, b - y);
+
+  canvas->FillRect({x, y, w, h}, color);
 }
 
 BEGIN_METADATA(Separator)