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I right-click both in Safari, opening new private viewing tabs then click PDF on each page.
The first one (1.) displays fine, but the second one (2.) either draws the page very slowly (tens of seconds) or fails completely. Partially generated text/images disappear if I scroll them off and back on to the screen. It's a little bit like erasing a white board.
However searching for text shows that it's there, just not rendered. Below are screenshots of my browser window and then the view of the saved PDF in Preview, searching for instances of "Astro".
I have seen several PDFs in the past that were somewhat slow (fractions of a second, just long enough to be visually disturbing when trying to skim a document) updating both in Safari and in Preview that I've never seen in much older versions of macOS or in Chrome, but I've always put off asking about it as it's been hard to reproduce in a way that can make a good Stack Exchange quesiton.
But here I've got a specific document and screen shots.
Question: Why is display of this PDF very slow and ultimately fails in Safari for me?
macOS 10.15.7, Safari 14.0
left: online PDF viewed in Safari, right: saved PDF viewed in Preview.
Update; very Interesting!
I can reproduce the scenario reliably on this laptop only.
I restarted the computer. I was using hotspot via USB before, now using hotspot via WiFi. Later I will try a different and potentially faster WIFi just for completeness.
Computer is always using AC power from the wall, battery is always fully charged.
Per @SolarMike's comment I've kept an eye on resources, so after restarting the only app I started is Safari.
- start Safari
- close window and then open a private browsing window
- log in to Stack Exchange
- navigate to this page
- right click link #2 to open in a new tab
- click on "Download: PDF" (which displays the PDF in the same tab)
- verified behavior; I can see the first page at first, scroll to third page, shows incomplete graphic, scroll back up shows pages now completely blank. Find "astro" shows the ghostly yellow rectangles on white screen.
- new observations on resources:
- Each time I go back to the original abstract page then click the PDF page, I can see the title page.
- When I scroll down to the top of the third page the graphic displays only partially.
- Scrolling back up now shows only blank pages
- I caught a 'This web page was reloaded because it was using significant energy.` message at the top of the Safari tab window. Aha!
- Investigating resources shows that although CPU usage is not high, the Energy display shows that Safari is working hard to do something!
- Sometimes, if I wait 20 or 30 seconds, the top of the PDF page will suddenly display again, and Energy drops down.
- If I start scrolling down, energy goes up and the "Disappearing act" begins again.
Could there be some energy-limiting setting...
...that I could change in order to see if Safari is being overly careful about energy usage at the expense of preventing me from seeing the PDF?
When PDF is blank, Energy usage by Safari is high. When it finally shows after 20 to 30 seconds, Safari energy usage drops:
CPU and memory usage is always relatively low and is certainly not taxing the system:
Even when PDF is blank, the "little window" that appears when hovering cursor over tab still renders the top of the page.
As soon as I started looking around in "About This Mac" the Bandcamp.app box appeared. I've never seen that happen before and I don't use this app.