Hidden tray icon even when not selected. #13296

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<!-- Describe the issue -->

While 'Minimize to the tray instead of the taskbar' and 'Minimize on close' is selected, and the Bitcoin Core Wallet window gets minimized, it sometimes disappears without showing in the system tray, forcing the user to signal termination via a task manager because maximizing isn't possible. Even when 'Hide tray icon' is unselected.

<!--- What behavior did you expect? -->

When minimizing or closing the Bitcoin Core Wallet window, it should show up in the system tray.

<!--- What was the actual behavior (provide screenshots if the issue is GUI-related)? -->

The Bitcoin Core Wallet window disappears, does not get shown in the tray, and with no way of maximizing or utilizing it

<!--- How reliably can you reproduce the issue, what are the steps to do so? -->

Minimize or close Bitcoin Core Wallet window while the following options are selected:

• 'Minimize to the tray instead of the taskbar'
• 'Minimize on close'

and this one unselected:

• 'Hide tray icon'

<!-- What version of Bitcoin Core are you using, where did you get it (website, self-compiled, etc)? -->

v0.16.0. https://bitcoin.org/nl/download. Linux 64 bit.

<!-- What type of machine are you observing the error on (OS/CPU and disk type)? -->

OS: Linux Mint 18.3 Sylvia (Xfce) (Compiz as window manager) CPU: Intel i7 3820 Disk: WD Caviar GP WD20000CSRTL2, 2TB

<!-- Any extra information that might be useful in the debugging process. -->

<!--- This is normally the contents of a `debug.log` or `config.log` file. Raw text or a link to a pastebin type site are preferred. -->

Is that still an issue?

I have no idea because I've since changed my distro.

I've reinstalled Linux Mint 19 Tara (Xfce) with the default window manager (Xfwm4 + Compositing), and the problem hasn't occurred since on Bitcoin v0.17.0.0.