Fix hanging problem while switching back a fully-folded document by using shortcut (Ctrl-TAB)

Use the _isFolding flag to avoid expensive redundant operations that cause an apparent hang when opening or switching tabs to a large fully-folded file.

The released version of Notepad++ uses the _isFolding flag to avoid expensive redundant operations when switching to a fully-folded tab using the mouse pointer.

However, opening a fully-folded file or switching to a fully-folded tab with the keyboard can produce a delay so long that Notepad++ appears to hang forever.

The hang is 100% reproducible and can be demonstrated in less than 60 seconds:

Download a deeply-nested file such as https://raw.githubusercontent.com/notepad-plus-plus/notepad-plus-plus/master/PowerEditor/src/Parameters.cpp or https://github.com/notepad-plus-plus/notepad-plus-plus/files/1555939/nppTest.zip
Open the file in Notepad++ and fold it using Alt-0
Create a new empty tab
Switch back and forth between tabs using the mouse, which is fast
Switch back and forth between tabs using the keyboard (Ctrl-Tab, Ctrl-Page(Up|Down), Ctrl-NumPad[0-9]), which will hang
This pull request applies the existing _isFolding flag to the forgotten keyboard and file open operations and prevents the hang caused by opening or switching tabs to a fully-folded file.

Fix #3996, fix #1128, fix #1806, fix #4871
Close #4867

PowerEditor/src/NppCommands.cpp

@@ -773,6 +773,7 @@ void Notepad_plus::command(int id)
 		{
 			const int index = id - IDM_VIEW_TAB1;
 			BufferID buf = _pDocTab->getBufferByIndex(index);
+			_isFolding = true;
 			if(buf == BUFFER_INVALID)
 			{
 				// No buffer at chosen index, select the very last buffer instead.
@@ -781,7 +782,10 @@ void Notepad_plus::command(int id)
 					switchToFile(_pDocTab->getBufferByIndex(last_index));
 			}
 			else
+			{
 				switchToFile(buf);
+			}
+			_isFolding = false;
 		}
 		break;
 
@@ -789,16 +793,21 @@ void Notepad_plus::command(int id)
 		{
 			const int current_index = _pDocTab->getCurrentTabIndex();
 			const int last_index = _pDocTab->getItemCount() - 1;
+			_isFolding = true;
 			if(current_index < last_index)
 				switchToFile(_pDocTab->getBufferByIndex(current_index + 1));
 			else
+			{
 				switchToFile(_pDocTab->getBufferByIndex(0)); // Loop around.
+			}
+			_isFolding = false;
 		}
 		break;
 
 		case IDM_VIEW_TAB_PREV:
 		{
 			const int current_index = _pDocTab->getCurrentTabIndex();
+			_isFolding = true;
 			if(current_index > 0)
 				switchToFile(_pDocTab->getBufferByIndex(current_index - 1));
 			else
@@ -806,6 +815,7 @@ void Notepad_plus::command(int id)
 				const int last_index = _pDocTab->getItemCount() - 1;
 				switchToFile(_pDocTab->getBufferByIndex(last_index)); // Loop around.
 			}
+			_isFolding = false;
 		}
 		break;
 
@@ -2899,6 +2909,7 @@ void Notepad_plus::command(int id)
 			nbDoc += viewVisible(SUB_VIEW)?_subDocTab.nbItem():0;
 
 			bool doTaskList = ((NppParameters::getInstance())->getNppGUI())._doTaskList;
+			_isFolding = true;
 			if (nbDoc > 1)
 			{
 				bool direction = (id == IDC_NEXT_DOC)?dirDown:dirUp;
@@ -2917,6 +2928,7 @@ void Notepad_plus::command(int id)
 					}
 				}
 			}
+			_isFolding = false;
 			_linkTriggered = true;
 		}
         break;

PowerEditor/src/NppIO.cpp

@@ -1556,7 +1556,9 @@ void Notepad_plus::loadLastSession()
 	const NppGUI & nppGui = nppParams->getNppGUI();
 	Session lastSession = nppParams->getSession();
 	bool isSnapshotMode = nppGui.isSnapshotMode();
+	_isFolding = true;
     loadSession(lastSession, isSnapshotMode);
+	_isFolding = false;
 }
 
 bool Notepad_plus::loadSession(Session & session, bool isSnapshotMode)