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Bug 1555511 Part 1: Make MVM::UpdateResolution clamp viewport zoom for the new display size later in the calculation. r=botond

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The existing math is attempting to adjust the display scale ratio to
prevent the new size from landing outside the min/max zoom bounds. This
introduces unwanted side effects that can be avoided by simply clamping
to min/max at the end. The remaining math correctly handles the case
where the zoom has ALREADY been clamped, which is the only important
case.

Differential Revision: https://phabricator.services.mozilla.com/D32908

--HG--
extra : moz-landing-system : lando
This commit is contained in:
Brad Werth 2019-05-31 22:10:28 +00:00
parent bf3d89574d
commit 8c5b5937fe
1 changed files with 24 additions and 22 deletions
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46
layout/base/MobileViewportManager.cpp
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@ -295,9 +295,14 @@ void MobileViewportManager::UpdateResolution(
// result of clamping to either the minimum or maximum zoom level
// allowed by the viewport. If we naively scale the zoom level with
// the change in the display width, we might be scaling one of these
// clamped values. What we really want to do is to make scaling of the
// zoom aware of these minimum and maximum clamping points, so that we
// keep display width changes completely reversible.
// previously clamped values. What we really want to do is to make
// scaling of the zoom aware of these minimum and maximum clamping
// points for the existing content size, so that we keep display
// width changes completely reversible.
// We don't consider here if we are scaling to a zoom value outside
// of our viewport limits, because we'll clamp to the viewport limits
// as a final step.
// Because of the behavior of ShrinkToDisplaySizeIfNeeded, we are
// choosing zoom clamping points based on the content size of the
@ -325,8 +330,7 @@ void MobileViewportManager::UpdateResolution(
float c(oldDisplaySize.width);
float d(newDisplaySize.width);
// For both oldDisplaySize and aDisplaySize, the values are in one of
// three "zones":
// The oldDisplaySize value is in one of three "zones":
// 1) Less than or equal to minZoomDisplaySize.
// 2) Between minZoomDisplaySize and maxZoomDisplaySize.
// 3) Greater than or equal to maxZoomDisplaySize.
@ -334,26 +338,24 @@ void MobileViewportManager::UpdateResolution(
// Depending on which zone each are in, the adjusted ratio is shown in
// the table below (using the a-b-c-d coding from above):
// d | 1 | 2 | 3 |
// c +---+---+---+
// | a | d | b |
// 1 | a | a | a |
// +---+---+---+
// | a | d | b |
// 2 | c | c | c |
// +---+---+---+
// | a | d | b |
// 3 | b | b | b |
// +---+---+---+
// c +---+
// | d |
// 1 | a |
// +---+
// | d |
// 2 | c |
// +---+
// | d |
// 3 | b |
// +---+
// Conveniently, the numerator is just d clamped to a..b, and the
// denominator is c clamped to a..b.
float numerator = clamped(d, a, b);
// Conveniently, the denominator is c clamped to a..b.
float denominator = clamped(c, a, b);
float adjustedRatio = numerator / denominator;
newZoom = Some(ScaleZoomWithDisplayWidth(
zoom, adjustedRatio, viewportSize, mMobileViewportSize));
float adjustedRatio = d / denominator;
CSSToScreenScale adjustedZoom = ScaleZoomWithDisplayWidth(
zoom, adjustedRatio, viewportSize, mMobileViewportSize);
newZoom = Some(ClampZoom(adjustedZoom, aViewportInfo));
}
}
} else { // aType == UpdateType::ContentSize