fix bug in line_gap.sml: when we convert a line number to an absolute index and travel leftwards, make sure we continue travelling leftwards when the current node is at the correct line number but the line number does not start at this node.

src/line_gap.sml

@@ -3308,8 +3308,16 @@ struct
           val prevIdx = curIdx - String.size shd
         in
           if findLine >= prevLine then
-            let val relativeLine = findLine - prevLine - 1
-            in Vector.sub (lhd, relativeLine) + prevIdx
+            let
+              val relativeLine = findLine - prevLine - 1
+            in
+              if relativeLine < 0 then
+                (* the line start is not at this node,
+                 * so we need to continue going left
+                 * to find what index the line start corresponds to *)
+                lineNumberToIdxLeft (findLine, prevIdx, prevLine, stl, ltl)
+              else
+                Vector.sub (lhd, relativeLine) + prevIdx
             end
           else
             lineNumberToIdxLeft (findLine, prevIdx, prevLine, stl, ltl)