8ead1439d8
The track-header (gtk) is packed with 1px box spacing. This is added at the top of each box (best seen by looking at the group-tab vs track-header alignment in the editor). However the actual track separator line on the canvas is at the bottom of each track, aligning with the top of the next track or automation-lane. The first track however lacks a separator at the top, which needs to be provided by the ruler. Once the canvas is scrolled this ruler/canvas separator overlaps with the bottom separator of each TAV. Using ruler Rectangle::set_outline had various issues. The outline extends outside the rectangle. It bled into the video-timeline instead of the canvas' top y-axis pixel. Also the separator was above the video-timeline, not above the canvas. Hence a dedicated separator Line is preferable. It also provides a consistent separator if no rulers are visible.
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