Commit 8c5b5937 authored by Brad Werth's avatar Brad Werth
Browse files

Bug 1555511 Part 1: Make MVM::UpdateResolution clamp viewport zoom for the new... 

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

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
parent bf3d8957

layout/base/MobileViewportManager.cpp

+25 −23
Original line number Diff line number Diff line
@@ -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 |

        //     +---+---+---+



        // 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);

        // c   +---+

        //     | d |

        // 1   | a |

        //     +---+

        //     | d |

        // 2   | c |

        //     +---+

        //     | d |

        // 3   | 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