Fix positioning of placeholder with multiline tabs

[forivall]

This fixes positioning of the placeholder in themes with multiline tabs (where I just updated mine to do this: https://atom.io/packages/adwaita-pro-ui)

[forivall]

Also provided by https://atom.io/packages/multirow-tabs

[lee-dohm]

I've added this to the queue for review. Would you mind updating it to resolve the conflicts?

[forivall]

Done! Twas a bit of work / complete rewrite, since y'all removed your use of jquery 😨 . Old version for ref: forivall@9895927

[winstliu]

Looks like you have a failing spec related to your change @forivall.
https://travis-ci.org/atom/tabs/builds/118844936#L182

[forivall]

Sorry for the long delay. @50Wliu @lee-dohm updated.

[winstliu]

Can you show a before/after image @forivall? I'm not sure I understand what the "placeholder" you're referring to is.

[forivall]

It's the blue thing.
Before:

After:

(These screenshots are using https://atom.io/packages/multirow-tabs
. My theme, which has multirow built-in, looks proper)

[Ben3eeE]

/cc: @simurai

[simurai]

A change in how flexbox works forced us to make the .placeholder element be relatively positioned (PR #384). And I think as a side effect, it also fixed this issue.

Should already be in the new Beta (1.12) if somebody wants to try.

[Ben3eeE]

Closing this one since it is already fixed by #384.