Fix `perspective-click` event for `"Treemap"` plugin

[texodus]

• Fixes #1949 and adds a test
• Fixes theme detection for Safari (and potentially other browsers) which caused the docs site to fail (in #1940 but this has already been deployed).
• Apparently new versions of Chromium do not report heap usage correctly without a special flag, so our leak test shave been silently failing since ???.
• Fixed missing --theme-bg0 variables in new template-based themes, which caused modal backgrounds to be transparent.
• Fixed a CSS regression which caused filter item test to overflow its container vertically.
• Reverted a change which suppresses log info at the cost of 20k.