Text Autosizing: prevent oscillation of font sizes during autosizing
https://bugs.webkit.org/show_bug.cgi?id=108205

Patch by Tim Volodine <timvolodine@chromium.org> on 2013-02-19
Reviewed by Kenneth Rohde Christiansen.

Source/WebCore:

On some websites autosized font-sizes oscillate due to layouts caused by
hovering or incremental page loading (and on other sites font sizes do
eventually stabilize, but it takes many layouts before they reach a steady
size). To prevent all these cases, we no longer allow the autosizing
multiplier to change after it has been set (to a value other than 1).

This won't always give exactly the same results, but testing on 2000 top
sites shows that this makes little difference in practice, and it prevents
these very jarring cases. As a happy side-effect, this speeds up layouts
as font sizes change less.

Test: fast/text-autosizing/oscillation-javascript-fontsize-change.html

* page/FrameView.cpp:
(WebCore::FrameView::setFrameRect):
* page/Settings.cpp:
(WebCore::Settings::setTextAutosizingFontScaleFactor):
* rendering/TextAutosizer.cpp:
(WebCore::TextAutosizer::recalculateMultipliers):
(WebCore):
(WebCore::TextAutosizer::processContainer):
* rendering/TextAutosizer.h:
(TextAutosizer):

LayoutTests:

Added overflow-y:hidden to some existing tests, since previously those tests
would start off with incorrect multipliers (because mainFrame->view()-layoutSize()
is initially 785 instead of 800 as ScrollView wrongly guesses a scrollbar will
be needed), and then the multipliers would get corrected on a subsequent layout.
Now that we don't allow the multiplier to change after being set, it needs to be
right first time.
Also added specific oscillation test triggered by javascript.

* fast/text-autosizing/constrained-height-body-expected.html:
* fast/text-autosizing/constrained-height-body.html:
* fast/text-autosizing/constrained-then-float-ancestors-expected.html:
* fast/text-autosizing/constrained-then-float-ancestors.html:
* fast/text-autosizing/constrained-then-position-fixed-ancestors-expected.html:
* fast/text-autosizing/constrained-then-position-fixed-ancestors.html:
* fast/text-autosizing/nested-em-line-height-expected.html:
* fast/text-autosizing/nested-em-line-height.html:
* fast/text-autosizing/oscillation-javascript-fontsize-change-expected.html: Added.
* fast/text-autosizing/oscillation-javascript-fontsize-change.html: Added.
* fast/text-autosizing/simple-paragraph-expected.html:
* fast/text-autosizing/simple-paragraph.html:
* fast/text-autosizing/span-child-expected.html:
* fast/text-autosizing/span-child.html:
* fast/text-autosizing/unwrappable-blocks-expected.html:
* fast/text-autosizing/unwrappable-blocks.html:
* fast/text-autosizing/unwrappable-inlines-expected.html:
* fast/text-autosizing/unwrappable-inlines.html:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@143356 268f45cc-cd09-0410-ab3c-d52691b4dbfc