This looks good at first, in Nightly, using the jsfiddle https://jsfiddle.net/fuh7c894/ , **but it gets terrible again** when a scrollbar has to appear (i.e. when I've clicked the last line enough times such that we have however many lines are needed to cause overflow). So, that situation (the presence of scrollbars) seems to defeat our optimization here -- I would guess because we reflow with two sizes -- with & without scrollbars -- and maybe that subtle change in conditions/viewport-width causes us to purge the cached value.
Bug 1682686 Comment 8 Edit History
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This looks good at first, in Nightly, using the jsfiddle https://jsfiddle.net/fuh7c894/ , **but it gets terrible again** when a scrollbar has to appear (i.e. when I've clicked the last line enough times such that we have however many lines are needed to cause overflow). So, that situation (the presence of scrollbars) seems to defeat our optimization here -- I would guess because we reflow with two sizes -- with & without scrollbars -- and maybe that subtle change in conditions/viewport-width causes us to purge the cached value. [EDIT: per my later down comments, I think this just has to do with the nesting level, and not really with the scrollbars]