How do I properly maximise a frame?

Question

When I maximise an Emacs frame, it fills almost, but not quite all the available space. I can still wiggle the frame around and a sliver of the desktop beneath is showing. Desktop environment is KDE 5.

Screenshot shows the right-most edge of the screen.

Answer 1

The original poster has indicated that setting frame-resize-pixelwise solved the issue:

— User Option: frame-resize-pixelwise

    If this option is nil, a frame's size is usually rounded to a multiple of the
current values of that frame's frame-char-height and frame-char-width whenever
the frame is resized. If this is non-nil, no rounding occurs, hence frame sizes
can increase/decrease by one pixel.

    Setting this variable usually causes the next resize operation to pass the
corresponding size hints to the window manager. This means that this variable
should be set only in a user's initial file; applications should never bind it
temporarily.

    The precise meaning of a value of nil for this option depends on the toolkit
used. Dragging the external border with the mouse is done character-wise provided
the window manager is willing to process the corresponding size hints. Calling
set-frame-size (see below) with arguments that do not specify the frame size as
an integer multiple of its character size, however, may: be ignored, cause a
rounding (GTK+), or be accepted (Lucid, Motif, MS-Windows).

    With some window managers you may have to set this to non-nil in order to
make a frame appear truly maximized or full-screen. 

Answer 2

Depending on exactly what you mean by maximized, you can use either toggle-frame-fullscreen or toggle-frame-maximized to achieve it through elisp (or M-x ...). Note, as @lawlist's answer points out, you may need to adjust frame-resize-pixelwise.

Here are the relevant docstrings. First, the "fullscreen" version:

(toggle-frame-fullscreen)

Toggle fullscreen mode of the selected frame. Enable fullscreen mode of the selected frame or disable if it is already fullscreen. Ignore window manager screen decorations. When turning on fullscreen mode, remember the previous value of the maximization state in the temporary frame parameter maximized. Restore the maximization state when turning off fullscreen mode.

Note that with some window managers you may have to set frame-resize-pixelwise to non-nil in order to make a frame appear truly fullscreen.

See also toggle-frame-maximized.

And the "maximized" version:

(toggle-frame-maximized)

Toggle maximization state of the selected frame. Maximize the selected frame or un-maximize if it is already maximized. Respect window manager screen decorations. If the frame is in fullscreen mode, don't change its mode, just toggle the temporary frame parameter maximized, so the frame will go to the right maximization state after disabling fullscreen mode.

Note that with some window managers you may have to set frame-resize-pixelwise to non-nil in order to make a frame appear truly maximized.

See also toggle-frame-fullscreen.