Mask Images Not Loaded While Printing

[G-Ambatte]

For some time now, there have been reports (see #4331) of masks not loading correctly in the Print Preview or printed output, and despite best efforts, there has not been a great deal of success in tracking down a root cause.

However, in the last few days I identified that the requests to download these masks is not being triggered until the Print event fires - and crucially, even if the mask is already cached, the response is not applied until AFTER the Print event has completed.

In short, to workaround this issue:

1. Click GET PDF.
2. Click Cancel.
3. Click GET PDF again.
4. Scroll through the Print Preview, once it loads. Generally, all masks will now have loaded correctly and you can proceed with the rest of the Print process as normal.

It appears that the workaround can fail if too much time passes between steps 1 and 3. If the masks have not applied correctly in step 4, return to step 2 and try again.

[G-Ambatte]

To investigate this issue, I created a test document that consists of seven blank pages, followed by four different full page masks (imageMaskEdges 2, 3, 4, and 8, selected at random) with the default dragon image from the snippet.

Opening Chrome's Dev Tools to the Network tab, I load the test document, with page loading completed at 1.92s.

After a sufficient period to be confident that no further activity will occur without being triggered, I clear the log - and then click Print.

I can see that the requests for the four mask images have been initiated, but all four are sitting at Pending in the Network tab.

Scrolling through the Print Preview, I can see that none of the images have been applied.

On clicking Cancel, I immediately see all four requests instantly complete.

Clicking GET PDF again, I see that there were NO new requests initiated, and all four image masks have been correctly applied,

HOWEVER

This appears to be a short lived fix. After a short amount of time has passed, clicking GET PDF initiates the image mask requests again, and once again, the masks do not appear in the Print Preview.

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It appears that scrolling through the document also causes the mask images to be loaded, however after spending some time with the pages out of the visible screen area, the mask images will once again be requested when the Print action is initiated.

[G-Ambatte]

Relates to Issue #4331.

[G-Ambatte]

Calculuschild has raised Chrome bug report for this issue; if you've been affected by it, please go +1 - https://issues.chromium.org/issues/438939374