Multi-process Firefox (Electrolysis / E10S) limitations

[dadler]

Thumbnail Zoom Plus mostly works in Firefox with Multi-Process Firefox enabled but there are some limitations.

See also:

• 160: Doesn't work with e10s (Electrolysis) -- multiprocess Firefox

• Firefox Bug 1068406 - "Thumbnail Zoom Plus" add-on does not work with e10s

General limitations:

• TZP works as long as Compatibility Shims are enabled (https://developer.mozilla.org/en-US/Firefox/Multiprocess_Firefox/Limitations_of_chrome_scripts#Compatibility_shims), which allow older-style coding to work but with some speed penalty. Thus it won't work if extensions.interposition.enabled is disabled in about:config.
• The shim requirement means TZP is probably slower than if it natively worked with E10s.
• There are many "Unsafe CPOW usage" warnings in the console, which are described here: https://developer.mozilla.org/en-US/Firefox/Multiprocess_Firefox/Cross_Process_Object_Wrappers#Safe_and_unsafe_CPOWs

Specific Feature Limitations:

• Incorrect TZP cursors when E10S is enabled. For example, it no longer shows the yellow error indicator when it can't load a pop-up's image. Cause is unknown.
• Add to History doesn't work when E10S is enabled. The underlying problem is that TZP can't query whether the window is in private-browsing mode, and so doesn't know whether it's safe to add to history. See TODO comment in common.js isPrivateBrowsing().
• Minor: Caption may show text which is supposed to be hidden. This happens since I had to disable calling window.getComputedStyle, which fails in E10S. Disabled even when E10S is off.