make alt-m on an empty prompt go to previous module

NEWS.md

@@ -138,7 +138,8 @@ Standard library changes
 
 * Tab complete hints now show in lighter text while typing in the repl. To disable
   set `Base.active_repl.options.hint_tab_completes = false` ([#51229]).
-* Meta-M with an empty prompt now returns the contextual module of the REPL to `Main`.
+* Meta-M with an empty prompt now returns the contextual module of the REPL to the previous
+  contextual module if there was one so that switching back and forth is simple. ([#51616], [#52670])
 
 #### SuiteSparse
 

stdlib/REPL/docs/src/index.md

@@ -578,8 +578,10 @@ Main
 
 It is possible to change this contextual module via the function
 `REPL.activate(m)` where `m` is a `Module` or by typing the module in the REPL
-and pressing the keybinding Alt-m (the cursor must be on the module name). The `Main` module can be "activated" with an empty prompt plus the keybinding. The
-active module is shown in the prompt (unless it is `Main`):
+and pressing the keybinding Alt-m with the cursor on the module name (Esc-m on MacOS).
+Pressing the keybinding on an empty prompt returns to the previously active
+module (i.e. `Main` after the first use). The active module is shown in the prompt
+(unless it is `Main`):
 
 ```julia-repl
 julia> using REPL
@@ -602,6 +604,10 @@ julia> Core<Alt-m> # using the keybinding to change module
 (Core) julia> <Alt-m> # going back to Main via keybinding
 
 julia>
+
+julia> <Alt-m> # going back to previously-active Core via keybinding
+
+(Core) julia>
 ```
 
 Functions that take an optional module argument often defaults to the REPL

stdlib/REPL/src/LineEdit.jl

@@ -1477,22 +1477,25 @@ end
 
 current_word_with_dots(s::MIState) = current_word_with_dots(buffer(s))
 
+previous_active_module = Main
+
 function activate_module(s::MIState)
     word = current_word_with_dots(s);
     mod = if isempty(word)
         edit_insert(s, ' ') # makes the `edit_clear` below actually update the prompt
-        Main
+        previous_active_module
     else
         try
             Base.Core.eval(Base.active_module(), Base.Meta.parse(word))
         catch
             nothing
         end
     end
-    if !(mod isa Module)
+    if !(mod isa Module) || mod == Base.active_module()
         beep(s)
         return
     end
+    global previous_active_module = Base.active_module()
     REPL.activate(mod)
     edit_clear(s)
 end