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
be0142ca03