commit be0142ca034c86234bd8f844e489b369dd112d3b
author Wyatt Allen <wyatta@google.com> Thu Aug 25 11:31:42 2016 -0700
committer Wyatt Allen <wyatta@google.com> Thu Aug 25 11:47:17 2016 -0700
tree 87e8d2f4674dd0c60c23c004d0ae00e55a023e7a
parent fc090c8f3698ca4d10b69bfbb8da3835b351a73e

Improves visible tabs rendering

This change reverts visible tabs to use the » character. A few different
approaches have been used for rendering these tab indicators:

I:  Before the Annotation Pipeline, tab indicators were configured by a
    class that was optionally applied to tab elements by the diff
    builder. The class was guarded by the show_tabs preference and a CSS
    rule created a `::before` pseudo element to insert the character.
    (Thus also preventing the » from being copyable text.)

II: When the Annotation Pipeline was implemented, the ordering of layers
    called for intraline difference elements being rendered *inside* tab
    indicators. As a result, the » indicator would sometimes have a
    different background than the intraline difference, looking sloppy.

    To solve this, the pseudo element was positioned using absolute,
    allowing the pseudo element to consume no horizontal space and and
    the intraline background to extend across the entire tab.

III:When performance tuning, it was discovered that the
    absolute-positioned tab indicators were positioned incorrectly when
    GPU acceleration was hinted for the diff, so the containing tab
    elements were made relative.

IV: Continuing performance tuning, the tab indicators seemed to slow
    scrolling on very large diffs with tabs. It was mistakenly assumed
    (by me) that this was related to the pseudo-elements when it was
    actually caused by the thousands of positioning contexts they
    created using relative and absolute.

    Instead they were modified to use strike-through instead of a pseudo
    element, which was more-performant, but less-usable (it looked bad).

With this change, we roll-back the clock a little and solve a core
problem: namely that tab indicators were not annotated inside the
intraline differences. Fixing that, positioning tricks are no-longer
needed to make the background look right.

To do this, we decouple the tab elements (which control tab width) from
the tab indicators (which optionally make tabs visible). This is done
using an annotation layer that inserts tab indicator elements wherever
a tab character is used in the source content, and it does so after
intraline differences have been applied.

Bug: Issue 4441
Change-Id: I4fea2a3a20a7039bfeb746bd1e1c1004e43c4801