Ken, Thank you for your reply. I thought I sent a reply to the list but apparently not. We were able to discover the cause of the problem and a fix. The default XQuartz Pasteboard preference was: Update Pasteboard when CLIPBOARD changes: checked If this box is unchecked, cutting and pasting works. This did lead to different problem, however, about the key mappings for cutting and pasting, which I posted in a followup post, which I'll repeat here. ---------- Begin forwarded message ---------- Dear XQuartz developers, I have a class where the majority of students are using Macs and XQuartz to interact with an app running on Linux. They are developing code with the app, so are doing a lot of copying and pasting. The students are used to using Cmd-C and Cmd-V to copy and paste, but the app expects Ctrl-C and Ctrl-V. Googling how to map the keys led to your website, where I tried the solution offered at https://xquartz.macosforge.org/trac/ticket/398 which is to have the students use this .Xmodmap file clear mod2 clear control keycode 63 = Control_L keycode 67 = Control_L keycode 71 = Control_L add control = Control_L Unfortunately, this doesn't work. I'm sure this is a commonly desired functionality and there must be a solution. Could you please tell me how to accomplish this key mapping. These are our XQuartz Pasteboard settings: Enable syncing: y Update Pasteboard when CLIPBOARD changes: n Update CLIPBOARD when Pasteboard changes: y Update PRIMARY (middle-click) when Pasteboard changes: y Update Pasteboard immediately when new text is selected: n Many thanks, Bill *From: *Ken Thomases <ken@codeweavers.com> *Subject: **Re: [Xquartz-dev] Class unable to view Qt5 Linux app using XQuartz* *Date: *April 12, 2016 at 8:13:30 PM PDT *To: *Developer talk about Xquartz <xquartz-dev@lists.macosforge.org> *Reply-To: *Developer talk about Xquartz <xquartz-dev@lists.macosforge.org> What X error is Qt getting? Presumably, you'd need the Qt developers' help in learning that. Maybe Qt has a verbose logging setting. Which selection is it using? They asked about the ssh X11 forwarding configuration. That would be useful for us, too. Did you try installing xsel on the Linux system and running it with a Mac as the X server? What were the results? Regards, Ken