gui2/ttext_: Add a public method for setting the selection range

Unlike the internal (protected) methods, this makes sure to call
update_canvas() when done so that e.g. text_box's highlight rectangle
is properly updated on the canvas with the right variables so that it's
drawn after we're done handling events.

src/gui/widgets/text.cpp

@@ -23,6 +23,8 @@
 
 #include "utils/functional.hpp"
 
+#include <limits>
+
 #define LOG_SCOPE_HEADER get_control_type() + " [" + id() + "] " + __func__
 #define LOG_HEADER LOG_SCOPE_HEADER + ':'
 
@@ -197,6 +199,38 @@ void ttext_::set_selection_length(const int selection_length)
 	}
 }
 
+void ttext_::set_selection(size_t start, int length)
+{
+	const size_t text_size = text_.get_length();
+
+	if(start >= text_size) {
+		start = text_size;
+	}
+
+	if(length == 0) {
+		set_cursor(start, false);
+		return;
+	}
+
+	// The text pos/size type differs in both signedness and size with the
+	// selection length. Such is life.
+	const int sel_start = std::min<size_t>(start, std::numeric_limits<int>::max());
+	const int sel_max_length = std::min<size_t>(text_size - start, std::numeric_limits<int>::max());
+
+	const bool backwards = length < 0;
+
+	if(backwards && -length > sel_start) {
+		length = -sel_start;
+	} else if(!backwards && length > sel_max_length) {
+		length = sel_max_length;
+	}
+
+	set_selection_start(start);
+	set_selection_length(length);
+
+	update_canvas();
+}
+
 void ttext_::set_state(const tstate state)
 {
 	if(state != state_) {

src/gui/widgets/text.hpp

@@ -90,6 +90,28 @@ class ttext_ : public tcontrol
 		text_changed_callback_ = cb;
 	}
 
+	/**
+	 * Sets or clears the text selection.
+	 *
+	 * Setting the selection range will re-position the cursor depending on the
+	 * selection direction, specified by the length's sign. Selecting beyond the
+	 * start or end of the text is safe; the final selection will be limited
+	 * accordingly.
+	 *
+	 * @note The range extents are measured in Unicode characters, not bytes.
+	 *       Using byte offsets may produce unexpected results depending on the
+	 *       text contents.
+	 *
+	 * @param start               Start offset, in characters.
+	 * @param length              Selection length, in characters. If zero, the
+	 *                            current selection is canceled. If negative, the
+	 *                            selection extends towards the start of the text
+	 *                            and the cursor will be re-positioned in that
+	 *                            direction as well; otherwise the selection and
+	 *                            cursor extend towards the end.
+	 */
+	void set_selection(size_t start, int length);
+
 protected:
 	/**
 	 * Moves the cursor to the end of the line.