Enable native clipboard provider by default

After successfuly loading the shim call GuiClipboard to enable the
native GUI clipboard.

- A new command GuiClipboard enables/disables the clipboard.
- Internally the Shim keeps the state of the setting and sets autocommands
  to reconfigure the clipboard on UI attachment.

The old function GuiClipboard is kept for compatibility.

ref #298

src/gui/runtime/doc/nvim_gui_shim.txt

@@ -86,6 +86,22 @@ GuiWindowOpacity	Set window opacity. Takes a single argument,
 				a double in the range 0.0 to 1.0 (fully opaque).
 				This capability might not be supported in some platforms.
 
+								*GuiClipboard*
+GuiClipboard	Enable or disable the native GUI clipboard.
+		When enabled, neovim will use the GUI as its clipboard
+		provider.
+
+		To enable,
+>
+			GuiClipboard 1
+<
+		to disable pass 0 to the command
+>
+			GuiClipboard 0
+<
+		After disabling the GUI clipboard, neovim will select
+		a clipboard provider from the available options.
+
 ==============================================================================
 2. GUI variables
 
@@ -199,6 +215,7 @@ menu can be mapped to right click events in ginit.vim, e.g.
 	nnoremap <silent><RightMouse> :call GuiShowContextMenu()<CR>
 	inoremap <silent><RightMouse> <Esc>:call GuiShowContextMenu()<CR>
 	vnoremap <silent><RightMouse> :call GuiShowContextMenu()<CR>gv
+<
 
 ==============================================================================
 4. Internals

src/gui/runtime/plugin/nvim_gui_shim.vim

@@ -132,6 +132,8 @@ function GuiName()
 	return get(info.client, 'name', '')
 endfunction
 
+let s:ui_clipboard_enabled = 0
+
 function s:ui_has_clipboard(idx, ui_info)
 	if has_key(a:ui_info, 'chan') == 0
 		return 0
@@ -148,45 +150,99 @@ function s:ui_has_clipboard(idx, ui_info)
 	endif
 endfunction
 
-"Enable a GUI provided clipboard
-function GuiClipboard()
+function s:reload_clipboard_provider()
+	" We need to reload the neovim clipboard provider here so it picks up on
+	" g:clipboard. In older versions of neovim (<=0.3.8) the provider would
+	" short circuit if a working clipboard was not available. After 0.3.8
+	" the provider should not short circuit, and unsetting
+	" g:loaded_clipboard_provider will enable a full reload of the provider.
+	"
+	" TLDR; source this to reinitialize the clipboard provider, this may not
+	" work
+	unlet! g:loaded_clipboard_provider
+	runtime autoload/provider/clipboard.vim
+endfunction
+
+function s:disable_custom_clipboard()
+	if exists("g:clipboard")
+		unlet g:clipboard
+	endif
+	call s:reload_clipboard_provider()
+endfunction
+
+"Enable a GUI clipboard
+function s:SetupGuiClipboard(silent)
 	if !has("nvim-0.3.2")
-		echoerr "UI clipboard requires nvim >=0.3.2"
+		if a:silent == 0
+			echoerr "UI clipboard requires nvim >=0.3.2"
+		endif
 		return
 	endif
 
 	let uis = nvim_list_uis()
 	call filter(uis, funcref('s:ui_has_clipboard'))
 	if len(uis) == 0
-		echoerr "No UIs with clipboard support are attached"
+		if a:silent == 0
+			echoerr "No UIs with clipboard support are attached"
+		end
+		call s:disable_custom_clipboard()
 		return
 	endif
 	let ui_chan = uis[-1].chan
 
-    let g:clipboard = {
-          \   'name': 'custom',
-          \   'copy': {
-          \      '+': {lines, regtype -> rpcnotify(ui_chan, 'Gui', 'SetClipboard', lines, regtype, '+')},
-          \      '*': {lines, regtype -> rpcnotify(ui_chan, 'Gui', 'SetClipboard', lines, regtype, '*')},
-          \    },
-          \   'paste': {
-          \      '+': {-> rpcrequest(ui_chan, 'Gui', 'GetClipboard', '+')},
-          \      '*': {-> rpcrequest(ui_chan, 'Gui', 'GetClipboard', '*')},
-          \   },
-          \ }
+	let g:clipboard = {
+		  \   'name': 'custom',
+		  \   'copy': {
+		  \      '+': {lines, regtype -> rpcnotify(ui_chan, 'Gui', 'SetClipboard', lines, regtype, '+')},
+		  \      '*': {lines, regtype -> rpcnotify(ui_chan, 'Gui', 'SetClipboard', lines, regtype, '*')},
+		  \    },
+		  \   'paste': {
+		  \      '+': {-> rpcrequest(ui_chan, 'Gui', 'GetClipboard', '+')},
+		  \      '*': {-> rpcrequest(ui_chan, 'Gui', 'GetClipboard', '*')},
+		  \   },
+		  \ }
+
+	call s:reload_clipboard_provider()
+endfunction
 
-	" We need to reload the neovim clipboard provider here so it picks up on
-	" g:clipboard. In older versions of neovim (<=0.3.8) the provider would
-	" short circuit if a working clipboard was not available. After 0.3.8
-	" the provider should not short circuit, andunsetting
-	" g:loaded_clipboard_provider will enable a full reload of the provider.
-	"
-	" TLDR; source this to reinitialize the clipboard provider, this may not
-	" work
-    unlet! g:loaded_clipboard_provider
-    runtime autoload/provider/clipboard.vim
+" For compatibility with an earlier version
+function GuiClipboard()
+	call s:SetupGuiClipboard(0)
+endfunction
+
+" Enable/Disable the GUI clipboard
+function s:GuiClipboardSet(enable)
+	if a:enable == 0
+		let s:ui_clipboard_enabled = 0
+		call s:disable_custom_clipboard()
+	elseif s:ui_clipboard_enabled == 1
+		" clipboard already enabled
+	else
+		if exists("g:clipboard")
+			echoerr "A custom g:clipboard is already configured"
+		endif
+
+		call s:SetupGuiClipboard(0)
+		let s:ui_clipboard_enabled = 1
+	endif
+endfunction
+command! -nargs=1 GuiClipboard call s:GuiClipboardSet(<args>)
+
+" If enabled reconfigure the GUI clipboard
+function s:UpdateGuiClipboard()
+	if s:ui_clipboard_enabled == 1
+		call s:SetupGuiClipboard(1)
+	endif
 endfunction
 
+" When a UI attaches/detaches try to reconfigure the GUI
+" clipboard
+augroup GuiClipboard
+	autocmd!
+	autocmd UIEnter * :call s:UpdateGuiClipboard()
+	autocmd UILeave * :call s:UpdateGuiClipboard()
+augroup END
+
 " Directory autocommands for Treeview
 augroup guiDirEvents
     autocmd!

src/gui/shell.cpp

@@ -273,6 +273,8 @@ void Shell::setAttached(bool attached)
 
 		auto req_shim = m_nvim->api0()->vim_command("runtime plugin/nvim_gui_shim.vim");
 		connect(req_shim, &MsgpackRequest::error, this, &Shell::handleShimError);
+		connect(req_shim, &MsgpackRequest::finished, this, &Shell::handleShimLoad);
+
 		auto gviminit = qgetenv("GVIMINIT");
 		if (gviminit.isEmpty()) {
 			auto req_ginit = m_nvim->api0()->vim_command("runtime! ginit.vim");
@@ -1869,6 +1871,18 @@ void Shell::handleShimError(quint32 msgid, quint64 fun, const QVariant& err)
 	qDebug() << "GUI shim error " << err;
 }
 
+void Shell::handleShimLoad(quint32 msgid, quint64 fun, const QVariant& resp)
+{
+	// Enable native clipboard, if api version 6 is available
+	auto api6 = m_nvim->api6();
+	if (api6) {
+		qDebug() << "Enabling native clipboard";
+		QVariantList args;
+		auto req = api6->nvim_command("GuiClipboard 1");
+		connect(req, &MsgpackRequest::error, this, &Shell::handleShimError);
+	}
+}
+
 void Shell::handleGetBackgroundOption(quint32 msgid, quint64 fun, const QVariant& val)
 {
 	const QString mode{ val.toString() };

src/gui/shell.h

@@ -113,6 +113,7 @@ protected slots:
 	void updateClientInfo();
 	void handleGinitError(quint32 msgid, quint64 fun, const QVariant& err);
 	void handleShimError(quint32 msgid, quint64 fun, const QVariant& err);
+	void handleShimLoad(quint32 msgid, quint64 fun, const QVariant& resp);
 	void handleGetBackgroundOption(quint32 msgid, quint64 fun, const QVariant& val);
 
 protected: