From a6100700385437dfb0306ac8dbef9c619c25a50b Mon Sep 17 00:00:00 2001
From: Paul Licameli <paul.licameli@audacityteam.org>
Date: Wed, 8 Jun 2016 18:20:23 -0400
Subject: [PATCH] May always scroll before zero during transport when head is
 pinned...

... regardless of preferences, but if the Tracks preference is not checked,
don't stay scrolled left of zero after stopping.
---
 src/Project.cpp | 26 ++++++++++++++++++++++----
 src/Project.h   |  1 +
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/src/Project.cpp b/src/Project.cpp
index 271078dab..5cc178c1e 100644
--- a/src/Project.cpp
+++ b/src/Project.cpp
@@ -1594,9 +1594,27 @@ void AudacityProject::OnScrollRightButton(wxScrollEvent & event)
 }
 
 
+bool AudacityProject::MayScrollBeyondZero() const
+{
+   if (mViewInfo.bScrollBeyondZero)
+      return true;
+
+   if (GetScrubber().HasStartedScrubbing() ||
+       IsAudioActive()) {
+      if (mPlaybackScroller) {
+         auto mode = mPlaybackScroller->GetMode();
+         if (mode == PlaybackScroller::Mode::Centered ||
+             mode == PlaybackScroller::Mode::Right)
+            return true;
+      }
+   }
+
+   return false;
+}
+
 double AudacityProject::ScrollingLowerBoundTime() const
 {
-   if (!mViewInfo.bScrollBeyondZero)
+   if (!MayScrollBeyondZero())
       return 0;
    const double screen = mTrackPanel->GetScreenEndTime() - mViewInfo.h;
    return std::min(mTracks->GetStartTime(), -screen / 2.0);
@@ -1708,7 +1726,7 @@ void AudacityProject::FixScrollbars()
    // may be scrolled to the midline.
    // May add even more to the end, so that you can always scroll the starting time to zero.
    const double lowerBound = ScrollingLowerBoundTime();
-   const double additional = mViewInfo.bScrollBeyondZero
+   const double additional = MayScrollBeyondZero()
       ? -lowerBound + std::max(halfScreen, screen - LastTime)
       : screen / 4.0;
 
@@ -2045,7 +2063,7 @@ void AudacityProject::OnScroll(wxScrollEvent & WXUNUSED(event))
    }
 
 
-   if (mViewInfo.bScrollBeyondZero) {
+   if (MayScrollBeyondZero()) {
       enum { SCROLL_PIXEL_TOLERANCE = 10 };
       if (std::abs(mViewInfo.TimeToPosition(0.0, 0
                                    )) < SCROLL_PIXEL_TOLERANCE) {
@@ -5418,7 +5436,7 @@ void AudacityProject::PlaybackScroller::OnTimer(wxCommandEvent &event)
       }
       viewInfo.h =
          viewInfo.OffsetTimeByPixels(viewInfo.h, deltaX, true);
-      if (!viewInfo.bScrollBeyondZero)
+      if (!mProject->MayScrollBeyondZero())
          // Can't scroll too far left
          viewInfo.h = std::max(0.0, viewInfo.h);
       trackPanel->Refresh(false);
diff --git a/src/Project.h b/src/Project.h
index 74cb4e54e..f485f1bd8 100644
--- a/src/Project.h
+++ b/src/Project.h
@@ -407,6 +407,7 @@ class AUDACITY_DLL_API AudacityProject final : public wxFrame,
 
    void SafeDisplayStatusMessage(const wxChar *msg);
 
+   bool MayScrollBeyondZero() const;
    double ScrollingLowerBoundTime() const;
    // How many pixels are covered by the period from lowermost scrollable time, to the given time:
    // PRL: Bug1197: we seem to need to compute all in double, to avoid differing results on Mac