[ Mojave WK1 ] Some Image tests are flakey failures and are failing in tandem with zoomed in or blank image results

[Truitt Savell]

There seems to be a fair amount of image tests that are flakey failures due to a zoomed in or blank rendered result. These tests also appear to be failing in tandem in small groups. These tests are not all part of the same directory but are displaying very similar failures. 

Bugs:
https://bugs.webkit.org/show_bug.cgi?id=193107
https://bugs.webkit.org/show_bug.cgi?id=193091
https://bugs.webkit.org/show_bug.cgi?id=192881

[Truitt Savell]

another set of tests appear to exhibiting this issue.

https://bugs.webkit.org/show_bug.cgi?id=193116

[Truitt Savell]

It appears that Image tests on Mojave WK1 in general have this zooming or blank issue. 

Here is a run with 15 failing image tests with many of them showing a zoomed in result:
https://build.webkit.org/results/Apple%20Mojave%20Release%20WK1%20(Tests)/r239610%20(1925)/results.html

[Radar WebKit Bug Importer]

<rdar://problem/47049667>

[Shawn Roberts]

Another set of tests exhibiting this issue.

https://build.webkit.org/results/Apple%20Mojave%20Release%20WK1%20(Tests)/r241018%20(2754)/results.html

[Shawn Roberts]

Also:

https://build.webkit.org/results/Apple%20Mojave%20Release%20WK1%20(Tests)/r241021%20(2756)/results.html

[Truitt Savell]

Set of 12 newer failure:
https://build.webkit.org/results/Apple%20Mojave%20Release%20WK1%20(Tests)/r245285%20(4955)/results.html

Set of 5:
https://build.webkit.org/results/Apple%20Mojave%20Release%20WK1%20(Tests)/r245210%20(4919)/results.html

Set of 10:
https://build.webkit.org/results/Apple%20Mojave%20Debug%20WK1%20(Tests)/r245219%20(3433)/results.html

[Said Abou-Hallawa]

This bug is happening because of a bug in CGWindowListCreateImage() in which it returns the wrong size snapshot when changing the window deviceScaleFactor from 2 to 1.

When running DRT, the bug happens only when running a hidpi test followed by a composite layout test. To fake the hipdi the deviceScaleFactor is set manually to 2. Before running a new test, the deviceScaleFactor has to be set back to 1. But this does not make CGWindowListCreateImage() works correctly in this case.

When createBitmapContextFromWebView() calls CGContextDrawImage(), only the bottom left quarter snapshot is drawn from the snapshot image to the bitmapContext. Most of the layout tests do not draw in this part of the WebView. So the result is seen as blank.

Like what we did in WebViewImpl::takeViewSnapshot() as a workaround for <rdar://problem/17084993>, we have to do similar workaround for DRT function createBitmapContextFromWebView().

[Said Abou-Hallawa]

Created attachment 374154 [details]
Patch

[WebKit Commit Bot]

Comment on attachment 374154 [details]
Patch

Clearing flags on attachment: 374154

Committed r247466: <https://trac.webkit.org/changeset/247466>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.