Add a horizontal position bar to flame graph.

Now that the ruler just shows the delta time across the view it doesn't
indicate where the view is currently looking.
The new position bar fills this role to allow orientating oneself.

manual/tracy.tex

@@ -4463,7 +4463,7 @@ The default sorting order of the zones on a flame graph \emph{approximates} the
 
 You can use an alternative sorting method by enabling the \emph{Sort by time} option. This will place the most time-consuming zones first (to the left) on the graph.
 
-You can navigate the flame graph using the mouse. Use the mouse wheel to zoom in and out around the mouse pointer. Pressing the \keys{\ctrl} key makes zooming more precise, while pressing the \keys{\shift} key makes it faster. Dragging with the \RMB{}~right mouse button pans the graph horizontally and vertically.
+You can navigate the flame graph using the mouse. Use the mouse wheel to zoom in and out around the mouse pointer. Pressing the \keys{\ctrl} key makes zooming more precise, while pressing the \keys{\shift} key makes it faster. Dragging with the \RMB{}~right mouse button pans the graph horizontally and vertically. The bar below the time ruler shows the current horizontal view position and can be dragged to pan the graph.
 
 Similar to the statistics window (section~\ref{statistics}), the flame graph can operate in two modes: \emph{\faSyringe{}~Instrumentation} and \emph{\faEyeDropper{}~Sampling}. In the instrumentation mode, the graph represents the zones you put in your program. In the sampling mode, the graph is constructed from the automatically captured call stack data (section~\ref{sampling}).
 

profiler/src/profiler/TracyView_FlameGraph.cpp

@@ -740,6 +740,75 @@ void View::DrawFlameGraphHeader( int64_t vStart, int64_t vEnd, uint64_t period )
     }
 }
 
+static void DrawFlameGraphHorizontalPosition( int64_t& vStart, int64_t& vEnd, int64_t totalSpan, uint64_t period )
+{
+    assert( vStart < vEnd );
+    assert( totalSpan > 0 );
+    assert( period > 0 );
+
+    const auto wpos = ImGui::GetCursorScreenPos();
+    const auto w = ImGui::GetContentRegionAvail().x;
+    const auto scale = GetScale();
+    const auto h = std::max( 8.f * scale, 8.f );
+
+    if( w <= 0 )
+    {
+        ImGui::Dummy( ImVec2( 0, h ) );
+        return;
+    }
+
+    ImGui::InvisibleButton( "##flameHorizontalPosition", ImVec2( w, h ) );
+    const auto hover = ImGui::IsItemHovered();
+    const auto active = ImGui::IsItemActive();
+    auto draw = ImGui::GetWindowDrawList();
+
+    const auto fullSpan = float( totalSpan );
+    const auto x0 = wpos.x + w * ( float( vStart ) / fullSpan );
+    const auto x1 = wpos.x + w * ( float( vEnd ) / fullSpan );
+    auto thumbX0 = std::max( wpos.x, std::min( wpos.x + w, x0 ) );
+    auto thumbX1 = std::max( thumbX0, std::min( wpos.x + w, x1 ) );
+    const auto minThumbWidth = std::max( 9.f * scale, 9.f );
+    if( thumbX1 - thumbX0 < minThumbWidth )
+    {
+        const auto center = ( thumbX0 + thumbX1 ) * 0.5f;
+        thumbX0 = center - ( minThumbWidth * 0.5f );
+        thumbX1 = center + ( minThumbWidth * 0.5f );
+        if( thumbX0 < wpos.x )
+        {
+            thumbX0 = wpos.x;
+            thumbX1 = thumbX0 + minThumbWidth;
+        }
+        else if( thumbX1 > wpos.x + w )
+        {
+            thumbX1 = wpos.x + w;
+            thumbX0 = thumbX1 - minThumbWidth;
+        }
+    }
+    const auto y0 = wpos.y + floor( h * 0.25f );
+    const auto y1 = wpos.y + ceil( h * 0.75f );
+
+    draw->AddRectFilled( ImVec2( wpos.x, y0 ), ImVec2( wpos.x + w, y1 ), 0x33888888 );
+    draw->AddRectFilled( ImVec2( thumbX0, y0 ), ImVec2( thumbX1, y1 ), active ? 0xCCFFFFFF : hover ? 0xAAFFFFFF : 0x66FFFFFF );
+
+    if( hover )
+    {
+        ImGui::BeginTooltip();
+        TextFocused( "View span:", TimeToString( ( vEnd - vStart ) * period ) );
+        ImGui::EndTooltip();
+    }
+
+    if( active && ImGui::IsMouseDragging( 0 ) )
+    {
+        const auto delta = ImGui::GetIO().MouseDelta.x;
+        const auto d = int64_t( delta * totalSpan / w );
+        if( d != 0 )
+        {
+            vStart += d;
+            vEnd += d;
+        }
+    }
+}
+
 static void MergeFlameGraph( std::vector<FlameGraphItem>& dst, std::vector<FlameGraphItem>&& src )
 {
     for( auto& v : src )
@@ -1125,13 +1194,11 @@ void View::DrawFlameGraph()
         {
             m_flameGraphViewEnd = zsz;
         }
-        else
-        {
-            ClampFlameGraphViewport( m_flameGraphViewStart, m_flameGraphViewEnd, zsz, ImGui::GetContentRegionAvail().x );
-        }
 
         const auto period = m_flameMode == 0 ? 1 : m_worker.GetSamplingPeriod();
         DrawFlameGraphHeader( m_flameGraphViewStart, m_flameGraphViewEnd, period );
+        DrawFlameGraphHorizontalPosition( m_flameGraphViewStart, m_flameGraphViewEnd, zsz, period );
+        ClampFlameGraphViewport( m_flameGraphViewStart, m_flameGraphViewEnd, zsz, ImGui::GetContentRegionAvail().x );
 
         ImGui::BeginChild( "##flameGraphBody", ImVec2( 0, 0 ), false, ImGuiWindowFlags_NoScrollWithMouse );
         const auto region = ImGui::GetContentRegionAvail();