Improve LBM/QML output buffer scroll behavior

Improves the LBM REPL and QMl output buffers in two ways:
1. Makes them only snap to the bottom when the view was already scrolled
   to the bottom. This makes is far easier to read output when connected
   to a device which prints stuff often.
2. Changes how the LBM REPL output is trimmed so that maintains the
   scroll position how you'd expect. The lack of this feature only
   became apparent after the first change.

Author: laxsjo

pages/pagelisp.cpp

@@ -202,29 +202,12 @@ void PageLisp::setVesc(VescInterface *vesc)
     ui->experimentPlot->setVesc(vesc);
 
     connect(mVesc->commands(), &Commands::lispPrintReceived, [this](QString str) {
-        ui->debugEdit->moveCursor(QTextCursor::End);
-        ui->debugEdit->insertPlainText(str + "\n");
-        ui->debugEdit->moveCursor(QTextCursor::End);
-
+        ui->debugEdit->doAtEndWithScroll([this, str](QTextCursor cursor) {
+            cursor.insertText(str + "\n");
+        });
+        
         int maxLines = QSettings().value("scripting/replMaxLines", 5000).toInt();
-        int removeLines = maxLines / 5;
-
-        if (ui->debugEdit->document()->lineCount() > maxLines) {
-            QString txt = ui->debugEdit->toPlainText();
-            auto lines = txt.split("\n");
-            if (lines.length() >= removeLines) {
-                QString shorter;
-                for (int i = removeLines;i < lines.length();i++) {
-                    shorter.append(lines.at(i));
-
-                    if (i != (lines.length() - 1)) {
-                        shorter.append("\n");
-                    }
-                }
-                ui->debugEdit->setText(shorter);
-                ui->debugEdit->moveCursor(QTextCursor::End);
-            }
-        }
+        ui->debugEdit->trimKeepingLastLines(maxLines);        
     });
 
     connect(mVesc->commands(), &Commands::lispRunningResRx, [this](bool ok) {

pages/pagescripting.cpp

@@ -226,11 +226,11 @@ void PageScripting::debugMsgRx(QtMsgType type, const QString msg)
         str = msg + "<br>";
     }
 
-    ui->debugEdit->moveCursor(QTextCursor::End);
-    ui->debugEdit->insertHtml("<font color=\"#4d7fc4\">" +
-                              QDateTime::currentDateTime().toString("yyyy-MM-dd hh:mm:ss: ") +
-                              "</font>" + str);
-    ui->debugEdit->moveCursor(QTextCursor::End);
+    ui->debugEdit->doAtEndWithScroll([this, str](QTextCursor cursor) {
+        cursor.insertHtml("<font color=\"#4d7fc4\">" +
+                          QDateTime::currentDateTime().toString("yyyy-MM-dd hh:mm:ss: ") +
+                          "</font>" + str);
+    });
 }
 
 void PageScripting::on_runButton_clicked()

widgets/vtextbrowser.cpp

@@ -20,6 +20,7 @@
 #include "vtextbrowser.h"
 #include <QDesktopServices>
 #include <QMouseEvent>
+#include <QScrollBar>
 
 VTextBrowser::VTextBrowser(QWidget *parent)
     : QTextEdit(parent)
@@ -59,3 +60,21 @@ void VTextBrowser::mouseMoveEvent(QMouseEvent *e)
 
     QTextEdit::mouseMoveEvent(e);
 }
+
+void VTextBrowser::trimKeepingLastLines(int maxAmount)
+{
+    int lines = document()->lineCount();
+    int lines_to_remove = lines - maxAmount;
+    if (lines_to_remove > 0) {
+        int scroll_before = verticalScrollBar()->value();
+        int scroll_max_before = verticalScrollBar()->maximum();
+
+        auto cursor = textCursor();
+        cursor.movePosition(QTextCursor::Start);
+        cursor.movePosition(QTextCursor::Down, QTextCursor::KeepAnchor, lines_to_remove);
+        cursor.removeSelectedText();
+
+        int scroll_max_delta = scroll_max_before - verticalScrollBar()->maximum();
+        verticalScrollBar()->setValue(scroll_before - scroll_max_delta);        
+    }
+}

widgets/vtextbrowser.h

@@ -21,6 +21,7 @@
 #define VTEXTBROWSER_H
 
 #include <QTextEdit>
+#include <QScrollBar>
 
 class VTextBrowser : public QTextEdit
 {
@@ -32,6 +33,33 @@ class VTextBrowser : public QTextEdit
     void mousePressEvent(QMouseEvent *e) Q_DECL_OVERRIDE;
     void mouseReleaseEvent(QMouseEvent *e) Q_DECL_OVERRIDE;
     void mouseMoveEvent(QMouseEvent *e) Q_DECL_OVERRIDE;
+    // Run `operation` with a cursor moved to the end of the document, making
+    // sure to scroll to the bottom if the operation changed the content's
+    // length and the view was at the bottom before.
+    // `operation` should be a callable taking a single `QTextCursor`.
+    template<typename F>
+    void doAtEndWithScroll(const F operation) {
+        // How far in pixels the scroll bar is allowed to be from the bottom
+        // while still being snapped to the bottom.
+        static const int bottom_margin = 30;
+
+        int scroll_before = verticalScrollBar()->value();
+        bool was_at_end = scroll_before >= verticalScrollBar()->maximum() - bottom_margin;
+
+        auto cursor = textCursor();
+        cursor.movePosition(QTextCursor::End);
+        operation(cursor);
+
+        if (was_at_end) {
+            verticalScrollBar()->setValue(verticalScrollBar()->maximum());
+        } else {
+            verticalScrollBar()->setValue(scroll_before);
+        }
+    };
+    // Remove the first lines so that only the last `maxAmount` of lines remain.
+    // The scroll position is maintained so that the user doesn't perceive the
+    // change.
+    void trimKeepingLastLines(int maxAmount);
 
 private:
     QString mLastAnchor;