For those reading this post and wondering "what is it like to 'close (Or Shrink) a panel' in Blender?":

Here is a sketch of what Blender looks like by default:

+-------------------------------+
| A |
+----------------------+--------+
| | B |
| +--------+
| C | |
| | |
| | D |
+----------------------+ |
| E | |
+----------------------+--------+

A, B, C, D, and E are areas - rectangular subregions of the application window. There are three rules about the arrangement of areas in Blender:

1. Areas are always rectangular
2. Every pixel in the window belongs to exactly one area
3. Every area is at least as tall as the height of a menu bar, and at least as wide as the height of a menu bar.

If you've used a tiling window manager before, then you're probably familiar with these rules and with the benefits of these rules. Areas always have a rectangular shape, so they are always laid out in a consistent way. Every pixel in the window belongs to an area, so no screen space is wasted. No part of the window has two areas, meaning that no area can ever be obscured by part of another area. And every area has a guaranteed minimum size, so you can never lose an area by accidentally shrinking its height or width to zero.

Because of these rules, some arrangements of areas are impossible. For example:

+-------------------------------+
| A |
+----------------------+--------+
| +---+ | B |
| | F | +--------+
| +---+ C | |
| | |
| | D |
+----------------------+ |
| E | |
+----------------------+--------+

This violates either the first or the second rule - area C must be rectangular (no holes!), and it cannot be covered by another area.

This is also not possible, because it would violate the same rules:

+-------------------------------+
| A |
+-------------------+-----------+
| | B |
| +--+--------+
| C | |
| | |
| | D |
+----------------------+ |
| E | |
+----------------------+--------+

This, however, is allowed:

+-------------------------------+
| A |
+----------------+--------------+
| | B |
| +--------------+
| C | |
| | |
| | D |
+----------------+ |
| E | |
+----------------+--------------+

You would accomplish this by placing you mouse cursor anywhere along the vertical line that separates C and E from B and D. The cursor would change to "<->". You would then hold down the left mouse button, drag to the left, and then release the left mouse button.

You could also do this:

+-------------------------------+
| A |
+----------------------+--------+
| | |
| | B |
| C | |
| | |
| +--------+
+----------------------+ D |
| E | |
+----------------------+--------+

...by placing your mouse cursor over the horizontal line that separates B from D, so that the cursor changes to the up-and-down arrows. Then, click the LMB, drag down, and release the LMB.

Besides changing the size of the currently-existing areas, you can also add new areas by dividing existing ones in two. For example:

+-------------------------------+
| A |
+-----------+----------+--------+
| | | B |
| | +--------+
| C | F | |
| | | |
| | | D |
+-----------+----------+ |
| E | |
+----------------------+--------+

This can be done by placing the mouse cursor over either the top or bottom edge of area C, so that the cursor changes to the up-and-down arrows. Then right-click. A menu will appear with two entries: "Split Area" and "Join Area". Select "split area", and a vertical line splitting area C will appear. As you move your cursor around, the line follows. If you move your cursor out of area C and into another area, the vertical line will follow into that area. When you click the LMB, area your cursor is over will be split into two, with the vertical border passing through where you clicked.

Horizontal divisions are, of course, also possible:

+-------------------------------+
| A |
+----------------------+--------+
| | B |
| +--------+
| C | D |
| | |
| +--------+
+----------------------+ F |
| E | |
+----------------------+--------+

Finally, you can reduce the number of areas by "joining" them. The only requirement for joining areas is that they must exactly share one edge. For example, this join is possible:

+-------------------------------+ +-------------------------------+
| A | | A |
+----------------------+--------+ +----------------------+--------+
| | B | | | |
| +--------+ | | |
| C | | -> | C | |
| | | | | B |
| | D | | | |
+----------------------+ | +----------------------+ +
| E | | | E | |
+----------------------+--------+ +----------------------+--------+

... because areas B and D exactly shared one edge. This join is done by placing your mouse cursor over the edge shared by areas B and D so that the cursor changes to the up-and-down arrows, right-clicking, and selecting "Join Area". Immediately, a gray overlay with an arrow in it will cover one of the two areas. If you move your cursor between areas B and D, the overlay will switch to the area your cursor is over.[0] This overlay makes it very clear which area will be enlarged to take over the other area. When the overlay covers area D, just click the left mouse button, and area D will be removed, and area B will be enlarged to fill its place.

One more note about joining areas. With the default layout, only area C can be joined to area E:

+-------------------------------+
| A |
+----------------------+--------+
| | B |
| +--------+
| C | |
| | |
| | D |
+----------------------+ |
| E | |
+----------------------+--------+

However, by moving the border between areas B, and D, we can get this layout:

+-------------------------------+
| A |
+----------------------+--------+
| | |
| | |
| C | B |
| | |
| | |
+----------------------+--------+
| E | D |
+----------------------+--------+

In this layout, either area C or area D can be joined with area E.

+-------------------------------+ +-------------------------------+ +-------------------------------+
| A | | A | | A |
+----------------------+--------+ +----------------------+--------+ +----------------------+--------+
| | | | | | | | |
| | | | | | | | |
| C | B | -> | | B | or | C | B |
| | | | C | | | | |
| | | | | | | | |
+----------------------+--------+ | +--------+ +----------------------+--------+
| E | D | | | D | | D |
+----------------------+--------+ +----------------------+--------+ +-------------------------------+