Created attachment 6297
Unable to drag pane-separators to the left or right in these two mail programs. The pane widths cannot be changed.
On Linux Mint 17.x
These are managed by the apps/toolkits the window manager is not involved with this.
when you say "apps", i think you mean the desktop software, geary and evolution in this case. Correct?
if so, i think it's not the app's fault, since it's affecting two completely differently apps.
when you say "toolkits", would that be gtk on mint 17?
(In reply to johny why from comment #3)
> when you say "toolkits", would that be gtk on mint 17?
I've checked, side panes in both geany and evolution work just fine in xfce with xfwm4-4.12.3 on Fedora 22 and Ubuntu 15.04, so it's something in your setup, maybe the theme?