Bug 5415 – Left border of selection highlight leaves behind a trail

• Status: VERIFIED
• Resolution: FIXED
• Component: Layout and Rendering

Summary: with certain fonts, contracting the selection by moving its start rightwards leaves trails of the 
highlight behind.

To reproduce: open the testcase in Safari. Click and drag in one motion from the end of the line to its 
beginning (extending the selection leftwards) and all the way back to the end (contracting the 
selection).

Expected result: no trails left behind.

Actual result: when the left border of the highlight is moving to the right, it leaves behind 1px-wide 
trails.

I think this happens because the clipping rect for the redraw is determined by WebCore using integral 
coordinates and drawing the highlight is done in WebTextRenderer using non-integral coordinates (or 
perhaps applying a different rounding rule).

Created attachment 4402 [details]
testcase

Created attachment 4448 [details]
extended testcase

Added RTL case.

Created attachment 4449 [details]
proposed patch

This patch fixes the trails bug in the CoreGraphics code path for LTR and RTL
text (highlighting for RTL text was entirely broken very recently) and in the
ATSUI code path for LTR text (highlighting and hit-testing RTL text in the
ATSUI path is broken and I'll open a separate bug for it. I already have a
patch).

This patch eliminates the single-character-range special case in the rounding
hack. One layout test is affected:
fast/text/international/bidi-explicit-embedding . The new results should
replace the current expected results.

[Darin Adler]

Comment on attachment 4449 [details]
proposed patch

Looks good. r=me

[David Harrison]

Committed.  Test case added as WebCore/manual-tests/drag_select_highlighting.html