Incorrect selection highlighting for ATSUI text when selected range is "CG-safe"

• Component: Layout and Rendering
• Status: VERIFIED FIXED

[Split off from bug 5166]
Summary: The selection highlight is partial or offset when the selection consists of "CG-safe" characters 
only but it is preceded by "need-ATSUI" characters.

To reproduce: double-click "dolor" in the testcase.

The position of the highlight rectangle depends on the width of all text before the selection, so the 
ATSUI code path should be used to draw the highlight even if the selected range itself does not require 
ATSUI.
The same applies to drawing text for the drag image when dragging a partial selection.

The fix for both is to change shouldUseATSU to always scan from the beginning. This should not affect 
layout and rendering performance, since run->from is zero anyway in all but the above two cases.

Created attachment 5133 [details]
testcase

Created attachment 5134 [details]
patch (incl. layout test)

Pixel mode required to detect regressions.

Comment on attachment 5134 [details]
patch (incl. layout test)

The code changes in this patch are identical to those in attachment 4965 [details].

[Darin Adler]

Comment on attachment 5134 [details]
patch (incl. layout test)

r=me