fx 81: LPB causes some background tabs to open with most of their main content hidden

[bughit]

• this started with fx 81
• I narrowed it down to LPB
• win7 x64 sp1, fx 81.0.0 x64, LPB 0.19, new profle
• load https://telegram.org/tour/channels
• middle click on the @Telegram link multiple times (6) to load background tabs
• after they all load, start switching to the tabs
• some of the tabs, don't have the main content
  
• until you scroll or resize them, at which point the main content appears
  
• if you disable LPB and restart (just in case), the problem goes away

[luben]

Hmm, that's strange. I will invesigate. Looks some bug with the rendering in v81 IMHO - else just scrolling will not fix it.