display: -ms-flexbox being removed despite compatibility mode

[viveleroi]

node v0.12.7, Mac OS X
clean-css v3.4.1

We're using flexbox in IE10 which supports an older flex standard. However, it seems that display: -ms-flexbox is being stripped from our css, despite any of the possible settings for compatibility mode.

Our original CSS:

display: -ms-flexbox;
-ms-flex-flow: row;
-ms-flex-wrap: wrap;

The final css (broken into newlines for this report)

-ms-flex-flow:row;
-ms-flex-wrap:wrap;

Other vendor prefixes for display are left alone.

[jakubpawlowicz]

@viveleroi can you reopen it with a larger excerpt of your CSS file as I can't figure out the bug from what you've provided.

[viveleroi]

You'll need to re-open this issue (I don't have access and would prefer not creating a new one).

I've trimmed our css down and am able to reproduce this issue with these exact steps:

test.css:

.columns {
  display: -ms-flexbox;
  -ms-flex-flow: row;
  -ms-flex-wrap: wrap;
  display: -webkit-flex;
  display: flex; }

Run cleancss -o test.min.css test.css

test.min.css:

.columns{-ms-flex-flow:row;-ms-flex-wrap:wrap;display:-webkit-flex;display:flex}

The display: -ms-flexbox; has gone missing.

One additional thing I've noticed, is that if I remove the two -ms-flex-* or the other two display attributes from the style block, the issue no longer presents. The output correctly contains display: -ms-flexbox;

[jakubpawlowicz]

So this is a duplicate of #290 - you need to use --skip-aggressive-merging switch for now.