Bug 1685756 - Part 3: Take default scale into account when disabling font inflation. r=hiro

Font inflation isn't required for pages whose viewport is narrow enough that we
don't have to zoom *out* when doing zoom-to-fit.

As the viewport scale is given as a CSSToScreenScale, this means that what we're
actually doing at the moment, though, is comparing the width of the viewport in
CSS pixels to the width of the screen in physical pixels, *without* taking the
default scale on a high-DPI device into account.

This issue affects neither pages using a "width=device-width" viewport (the
IsAutoSizeEnabled() check handles those), nor pages without an explicitly
specified viewport at all (the viewport info returns the minimum zoom level in
that case, i.e. 25 %, which would therefore require a device scale of > 4 in
order for us to erroneously turn off font inflation - even modern phones don't
seem to quite reach *that* sort of pixel density, e.g. a Pixel 5 only reports a
default scale of ~2.73.

For pages with an explicitly sized viewport this can however mean that we turn
off font inflation too early: Slashdot e.g. uses "width=1000", which means that
on a phone with a 720 px screen font inflation is correctly enabled, but on a
different phone squeezing 1080 px into the same physical screen width (i.e.
higher DPI, but *not* a physically larger screen), font inflation is suddenly
and incorrectly turned off.

Differential Revision: https://phabricator.services.mozilla.com/D122059

layout/base/PresShell.cpp

@@ -11472,8 +11472,10 @@ bool PresShell::DetermineFontSizeInflationState() {
       PixelCastJustification::LayoutDeviceIsScreenForBounds);
   nsViewportInfo vInf = GetDocument()->GetViewportInfo(screenSize);
 
-  if (vInf.GetDefaultZoom() >= CSSToScreenScale(1.0f) ||
-      vInf.IsAutoSizeEnabled()) {
+  CSSToScreenScale defaultScale =
+      mPresContext->CSSToDevPixelScale() * LayoutDeviceToScreenScale(1.0);
+
+  if (vInf.GetDefaultZoom() >= defaultScale || vInf.IsAutoSizeEnabled()) {
     return false;
   }
 

layout/reftests/font-inflation/reftest.list

@@ -70,9 +70,9 @@ test-pref(font.size.inflation.emPerLine,15) test-pref(font.size.inflation.forceE
 test-pref(font.size.inflation.emPerLine,15) test-pref(font.size.inflation.forceEnabled,true) test-pref(font.size.inflation.lineThreshold,0) == form-button-fixed-width.html form-button-fixed-width-ref.html
 test-pref(font.size.inflation.emPerLine,15) test-pref(font.size.inflation.forceEnabled,true) test-pref(font.size.inflation.lineThreshold,0) == form-button.html form-button-ref.html
 test-pref(font.size.inflation.emPerLine,15) test-pref(font.size.inflation.forceEnabled,true) test-pref(font.size.inflation.lineThreshold,0) == fixed-width-viewport-no-inflation.html fixed-width-viewport-no-inflation.html
-fails asserts(0-2) pref(layout.css.devPixelsPerPx,"0.5") test-pref(font.size.inflation.emPerLine,15) test-pref(font.size.inflation.forceEnabled,true) test-pref(font.size.inflation.lineThreshold,0) == fixed-width-viewport-no-inflation.html fixed-width-viewport-no-inflation.html
+asserts(0-2) pref(layout.css.devPixelsPerPx,"0.5") test-pref(font.size.inflation.emPerLine,15) test-pref(font.size.inflation.forceEnabled,true) test-pref(font.size.inflation.lineThreshold,0) == fixed-width-viewport-no-inflation.html fixed-width-viewport-no-inflation.html
 test-pref(font.size.inflation.emPerLine,15) test-pref(font.size.inflation.forceEnabled,true) test-pref(font.size.inflation.lineThreshold,0) == fixed-width-viewport-inflation.html fixed-width-viewport-inflation-ref.html
-fails skip-if(Android&&webrender||cocoaWidget) asserts(0-2) pref(layout.css.devPixelsPerPx,"2.0") test-pref(font.size.inflation.emPerLine,15) test-pref(font.size.inflation.forceEnabled,true) test-pref(font.size.inflation.lineThreshold,0) == fixed-width-viewport-inflation.html fixed-width-viewport-inflation-ref.html
+skip-if(Android&&webrender||cocoaWidget) asserts(0-2) pref(layout.css.devPixelsPerPx,"2.0") test-pref(font.size.inflation.emPerLine,15) test-pref(font.size.inflation.forceEnabled,true) test-pref(font.size.inflation.lineThreshold,0) == fixed-width-viewport-inflation.html fixed-width-viewport-inflation-ref.html
 
 # Ordinarily, reftests use a browser.viewport.desktopWidth of 800px, same as the
 # size of the reftest document.  The failure condition of the test below however