[Webkit-unassigned] [Bug 190805] New: Background flicker at edges while WKWebView size is being decreased

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=190805

            Bug ID: 190805
           Summary: Background flicker at edges while WKWebView size is
                    being decreased
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebKit2
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: jalkut at red-sweater.com

When a window containing a WKWebView is resized, the content area of the web view draws to completely fill the expected bounds when the size is getting larger, but seems to draw too small while shrinking the size of the window. This results in a kind of white background flicker that is more noticeable when there is a great contrast between the content, the white WKWebView background, and the windows behind the WKWebView-hosting window.

I noticed this issue particularly while I was debugging Dark Mode support in my Mac app, because the dark content in one of my screens creates a quite noticable white flickr on the edges where it occurs.

I am able to reproduce the problem using the r237327 Oct 22, 2018 nightly on Mojave 10.14 (18A391). One example for reproducing it:

1. Open this Data URL in the browser:

data:text/html;base64,PGhlYWQ+CjxzY3JpcHQ+CiAgICBmdW5jdGlvbiBjb250ZXh0TWVudSh3aGF0KSB7CiAgICBjb25zb2xlLmxvZyh3aGF0KTsKICAgIH0KICBkb2N1bWVudC5hZGRFdmVudExpc3RlbmVyKCJET01Db250ZW50TG9hZGVkIiwgZnVuY3Rpb24oZXZlbnQpIHsKICAgIGRvY3VtZW50LnF1ZXJ5U2VsZWN0b3IoJ2JvZHknKS5zZXRBdHRyaWJ1dGUoJ29uY29udGV4dG1lbnUnLCAiY29udGV4dE1lbnUod2hhdCk7Iik7CiB9KTsKPC9zY3JpcHQ+CjwvaGVhZD4KPGJvZHkgd2lkdGg9IjEwMCUiIGhlaWdodD0iMTAwJSIgYmdjb2xvcj0icmVkIj4KaGVsbG8KPC9ib2R5Pg==

2. Click the right edge of the browser window and pull to the right. Move slowly and quickly. The red background content keeps up at all times.

3. Now do the same thing, moving to the left, shrinking the width of the window. The faster you jerk the mouse the more noticeable a gap you will create between the drawing of the content and the edge of the web view.

Combined with the shortcoming that WKWebView does not expose a public API for setting background color, this makes it hard for me to present a polished looking resizable window that is backed by WKWebView content.

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