b3417b0e96
CM3 does not compute the height of line widgets if the widget is offscreen and has not been rendered yet. This allows rendering large documents faster, but means Y coordinates from one CM3 do not align common lines of code in the other CM3 instance. During scrolling determine the line visible at the top of the viewport. Rely on the LineMapper data to locate the common line either at this position, or the first common line before it. With the nearest prior common line in hand, compute the distance between the viewport top and this common line, and add that to the position of the common line on the other CM3. This should pixel align both CM3 instances even if other widgets are not properly measured. Using this alignment method the view scrolls almost "butter smooth" under Chrome on a modern MacBook Pro. It is difficult to tell there are two synchronized CM3 instances. Keep the 20ms/40ms timer running during scrolling. The timer adjusts the other CM3 every 20ms to ensure it is aligned and cancels after 40ms of no scrolling activity. Running the timer is a backup measure to ensure the panes do not fall out of sync. The 40ms cancel timer must be kept to "disconnect" the current scrolling event and allow the other CM3 to control a future scroll attempt. Most jank and artifacting during throw scrolling is now gone. This commit was tested on an edit of a 12,000 line text file containing an insertion at line 7, and another at 11,838, and 200 lines of context. Change-Id: I4d2305abcd16040a67a4d3534981d75dd2c3a5d9