Positions of elements in the initial flattened list in a split list

You can use

SplitBy[Range@Length@m1, m1[[#]] &]

There is a complicated trade-off between the speed and compact form in this case, so I have decided to post the version with Range, which I consider simple enough (comprehensible for new users) and second fast among the conterparts (at least, on my machine).

It is heavily based on @Mr.Wizard solution farsightedly provided by ChrisDegnen, so I do not claim originality:

dynS[p_] := Range @@@ Thread[{Accumulate@p - p + 1, Accumulate@p}]

---

Method

And after looking at @Mr.Wizard SparseArray solution I finally realize that we may use Listable attribute of Range to get even more compact version (this time I would prefer to keep pure function notation #). So this is favorite method for me (not mine :)!

dynSP[p_] := Range[# - p + 1, #]& @ Accumulate @ p

---

Timing benchmarking

I use the long list for benchmarking:

m1 = Flatten[Table[#, # + 1] & /@ RandomInteger[{1, 200}, 10^5]];
Length[m1]
(* 10156647 *)

And packed version later (second timing output in each method).

m1 = Developer`ToPackedArray[m1];

---

Range[Prepend[# + 1, 1], Append[#, Length @ m1]] & 
@ SparseArray[Differences @ m1]["AdjacencyLists"] // Length // RepeatedTiming
(* {0.403, 99530} *)
(* {0.274, 99489} *)

dynSP[Length /@ Split @ m1] // Length // RepeatedTiming
(* {0.476, 99439} *)
(* {0.626, 99439} *)

dynS[Length /@ Split @ m1] // Length // RepeatedTiming
(* {0.506, 99495} *)
(* {0.715, 99489} *)

Internal`PartitionRagged[Range[Length@m1], Length /@ Split@m1] // Length // RepeatedTiming
(* {0.589, 99495} *)
(* {0.78, 99489}  *)

dynP[Range@Length@m1, Length /@ Split @ m1] // Length // RepeatedTiming
(* {0.613, 99495} *)
(* {0.83, 99489}  *)

Module[{i = 1}, Replace[Split@m1, _ :> i++, {-1}]] // Length // RepeatedTiming
(* {3.845, 99439} *)
(* {4.1, 99439}   *)

Module[{i = 0}, Map[++i &, Split[m1], {-1}]] // Length // RepeatedTiming
(* {6.57, 99495} *)
(* {6.85, 99489} *)

SplitBy[Range @ Length @ m1, m1[[#]] &] // Length // RepeatedTiming
(* {24.6, 99495} *)
(* {25., 99489}  *)

Note: fastest function with SparseArray has been added a bit later so its result in terms of length is slightly different. The same is for the Module with Split version and my favorite DynSP.

Perhaps:

s = Split@m1;
Internal`PartitionRagged[Range[Length@m1], Length /@ s]