Mouse-based editing is what XNEdit is all about, and learning to use the more advanced features like secondary selections and primary selection dragging will be well worth your while.
If you don't have time to learn everything, you can get by adequately with just the left mouse button: Clicking the left button moves the cursor. Dragging with the left button makes a selection. Holding the shift key while clicking extends the existing selection, or begins a selection between the cursor and the mouse. Double or triple clicking selects a whole word or a whole line.
This section will make more sense if you also read the section called, "Selecting Text", which explains the terminology of selections, that is, what is meant by primary, secondary, rectangular, etc.
General meaning of mouse buttons and modifier keys:
Button 1 (left) Cursor position and primary selection
Button 2 (middle) Secondary selections, and dragging and
copying the primary selection
Button 3 (right) Quick-access programmable menu and pan
scrolling
Shift On primary selections, (left mouse button):
Extends selection to the mouse pointer
On secondary and copy operations, (middle):
Toggles between move and copy
Ctrl Makes selection rectangular or insertion
columnar
Alt* (on release) Exchange primary and secondary
selections.
The left mouse button is used to position the cursor and to make primary selections.
Click Moves the cursor
Double Click Selects a whole word
Triple Click Selects a whole line
Quad Click Selects the whole file
Shift Click Adjusts (extends or shrinks) the
selection, or if there is no existing
selection, begins a new selection
between the cursor and the mouse.
Ctrl+Shift+ Adjusts (extends or shrinks) the
Click selection rectangularly.
Drag Selects text between where the mouse
was pressed and where it was released.
Ctrl+Drag Selects rectangle between where the
mouse was pressed and where it was
released.
The right mouse button posts a programmable menu for frequently used commands.
Click/Drag Pops up the background menu (programmed
from Preferences -> Default Settings ->
Customize Menus -> Window Background).
Ctrl+Drag Pan scrolling. Scrolls the window
both vertically and horizontally, as if
you had grabbed it with your mouse.
The middle mouse button is for making secondary selections, and copying and dragging the primary selection.
Click Copies the primary selection to the
clicked position.
Shift+Click Moves the primary selection to the
clicked position, deleting it from its
original position.
Drag 1) Outside of the primary selection:
Begins a secondary selection.
2) Inside of the primary selection:
Moves the selection by dragging.
Ctrl+Drag 1) Outside of the primary selection:
Begins a rectangular secondary
selection.
2) Inside of the primary selection:
Drags the selection in overlay
mode (see below).
When the mouse button is released after creating a secondary selection:
No Modifiers If there is a primary selection,
replaces it with the secondary
selection. Otherwise, inserts the
secondary selection at the cursor
position.
Shift Move the secondary selection, deleting
it from its original position. If
there is a primary selection, the move
will replace the primary selection
with the secondary selection.
Otherwise, moves the secondary
selection to the cursor position.
Alt* Exchange the primary and secondary
selections.
While moving the primary selection by dragging with the middle mouse button:
Shift Leaves a copy of the original
selection in place rather than
removing it or blanking the area.
Ctrl Changes from insert mode to overlay
mode (see below).
Escape Cancels drag in progress.
Overlay Mode: Normally, dragging moves text by removing it from the selected position at the start of the drag, and inserting it at a new position relative to the mouse. When you drag a block of text over existing characters, the existing characters are displaced to the end of the selection. In overlay mode, characters which are occluded by blocks of text being dragged are simply removed. When dragging non-rectangular selections, overlay mode also converts the selection to rectangular form, allowing it to be dragged outside of the bounds of the existing text.
Mouse buttons 4 and 5 are usually represented by a mouse wheel nowadays. They are used to scroll up or down in the text window.
* The Alt key may be labeled Meta or Compose-Character on some keyboards. Some window managers, including default configurations of mwm, bind combinations of the Alt key and mouse buttons to window manager operations. In XNEdit, Alt is only used on button release, so regardless of the window manager bindings for Alt-modified mouse buttons, you can still do the corresponding XNEdit operation by using the Alt key AFTER the initial mouse press, so that Alt is held while you release the mouse button. If you find this difficult or annoying, you can re-configure most window managers to skip this binding, or you can re-configure XNEdit to use a different key combination.