Allow user to easily select one or more lines of source code to copy, to comment on them and to scroll the page

This is an umbrella issue to address several usability issues we currently have with adding comments, selecting source code and scrolling the page.

Problems reported by users:

• selecting source code opens the "Add comment" inline dialog; instead of this it should allow to copy code, also from multiple lines (CRUC-1218, CRUC-5890, CRUC-7294*, CRUC-7751)
• current "Alt+drag mouse" for code selection does not work on some Linux distributions (CRUC-5919)
• it should be possible to set the code viewport as active for scrolling purposes (CRUC-6332)

What we need

• selection of source code using a mouse without a need to hold Alt (or any other key)
• selection of multiple source code lines should select code only and not line numbers
• adding new comments
• adding new comments covering multiple source lines (also non-continuous ones)
• single click on source code should do nothing (i.e. just set a focus for scrolling)

How it could work (proposal)

• hovering mouse over left margin with line numbers would show the "<+>" button
  • clicking on it would show the "Add comment" dialog
  • pressing Ctrl/Cmd should allow to mark additional lines
• selecting source lines using a mouse would select source code
  • it would not pop-up the "Add comment" dialog
  • it would not select row numbers
• a single click on the source code would do nothing
  • it's to give a focus on the frame for scrolling

Workaround

A bug fix for copying source is available as a Copy Source for FishEye and Crucible plugin on Marketplace.