Bug 193908

Summary: "async" and "defer" script elements delay painting
Product: WebKit Reporter: darren.hebner
Component: Layout and RenderingAssignee: alan <zalan>
Status: NEW    
Severity: Normal CC: bfulgham, simon.fraser, webkit-bug-importer, zalan
Priority: P2 Keywords: InRadar
Version: Safari 12   
Hardware: iPhone / iPad   
OS: iOS 12   
Bug Depends on: 194168    
Bug Blocks:    
Attachments:
Description Flags
GIF showing the expected behaviour and the current behaviour in Safari none

darren.hebner
Reported 2019-01-28 07:40:07 PST
Created attachment 360341 [details] GIF showing the expected behaviour and the current behaviour in Safari I'm noticing that Safari does not render anything until all JavaScript has been downloaded and executed. This includes deferred and async scripts as well as scripts that appear just before the closing body tag. I've created a small site to demonstrate this behaviour: https://wkwebviewtest.surge.sh/ This demo site features a script tag with the "defer" attribute in the <head> of the document. This script is intentionally slow and takes ~5s to execute. When the script is finished, it appends a message to the page. The expected behaviour is for the HTML to render immediately since the pages JavaScript is deferred. To reproduce, visit the demo site in new window or tab. You will see that the browser waits until the script has finished executing (~5s) before it renders anything. It's worth noting that I'm seeing the same behaviour in all of these scenarios: - The script is in the <head> with the "defer" attribute - The script is in the <head> with the "async" attribute - The script is just before the closing </body> tag. This behaviour is detrimental to performance and goes against the benefits of using the defer attribute. Thank you in advance for your help!
Attachments
GIF showing the expected behaviour and the current behaviour in Safari (6.26 MB, image/gif)
2019-01-28 07:40 PST, darren.hebner 		no flags
Details
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Simon Fraser (smfr)
Comment 1 2019-01-28 11:00:44 PST
Zalan, does your new first paint stuff improve this?
Radar WebKit Bug Importer
Comment 2 2019-01-28 11:00:49 PST
<rdar://problem/47602939>
alan
Comment 3 2019-02-04 20:54:55 PST
first part of the fix: bug 194168
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