One possible solution to RT#120895

Deindents delete from the front.  Any spaces that are not
contributing to the length are deleted along with the tab
they are concealed by.

Spaces from tab explosions still get added to the end of
the remaining whitespace.  You can no longer expect
e.g. .indent(4).indent(-4) to roundtrip, but this will
consistently cause tabs to be moved leftward and deleted
when mixing spaces.

The example in the RT will work if $?TABSTOP is set to 4 first.
(We can't preserve visual consistency without requiring this.)

src/core/Str.pm

@@ -1346,15 +1346,29 @@ my class Str does Stringy { # declared in BOOTSTRAP
         warn "Asked to remove $outdent spaces, but the shortest indent is $common-prefix spaces"
             if $outdent > $common-prefix;
 
-        # Work backwards from the right end of the indent whitespace, removing
+        # Work forwards from the left end of the indent whitespace, removing
         # array elements up to # (or over, in the case of tab-explosion)
         # the specified outdent amount.
-        @lines.map({
+        @lines.map(-> $l {
             my $pos = 0;
-            while $_<indent-chars> and $pos < $outdent {
-                $pos += $_<indent-chars>.pop.value;
+            while $l<indent-chars> and $pos < $outdent {
+                if $l<indent-chars>.shift.key eq "\t" {
+                    $pos -= $pos % $?TABSTOP;
+                    $pos += $?TABSTOP;
+                } else {
+                    $pos++
+                }
+            }
+            if $l<indent-chars> and $pos % $?TABSTOP {
+                my $check = $?TABSTOP - $pos % $?TABSTOP;
+                $check = $l<indent-chars>[0..^$check].first-index({$_.key eq "\t"});
+                if $check.defined {
+                    $l<indent-chars>.shift for 0..$check;
+                    $pos -= $pos % $?TABSTOP;
+                    $pos += $?TABSTOP;
+                }
             }
-            $_<indent-chars>».key.join ~ ' ' x ($pos - $outdent) ~ $_<rest>;
+            $l<indent-chars>».key.join ~ ' ' x ($pos - $outdent) ~ $l<rest>;
         }).join;
     }