Cannot till on top/bottom half of the screen when the other half is already used by multiple windows

Describe the bug

When trying to till a window on the free top or bottom half of the screen while its other half is already used by multiple windows, the rectangle hint showing the target never suggests the entire free zone.

Steps To Reproduce

Try to get this layout:

===========================
|           |            |
|     A     |     B      |
|           |            |
|===========|============|
|                        |
|           C            |
|                        |
==========================

1. Till a window at the top-left corner of the screen (A)
2. Till a window at the top-right corner of the screen (B)
3. Grab a window while pressing Super, then hold Control and move the window around the center of the bottom half (C).

Here, the hint showing the tiling target should cover exactly the C area but it actually shows a column from the top to the bottom of the screen.

Trying a similar scenario horizontally does work as expected, e.g. with the target layout:

==========================
|           |            |
|     A     |            |
|           |            |
|===========|      C     |
|           |            |
|     B     |            |
|           |            |
==========================

Here, when moving the window around the center of the C area, the target will cover exactly that area.

The issue also happens with a smaller zone, as long as that zone has tiled windows below it, e.g.

==========================
|           |            |
|     A     |     E      |
|           |            |
|===========|============|
|           |     |      |
|     B     |  C  |  D   |
|           |     |      |
==========================

Here, assuming all zones are tiled windows except E, one can't till a window in the entire E area at once, one can only target a part of it.

In all scenarios, one first needs to tile the window in a part of the area they want, then extend the window by grabbing its edge and moving it while pressing Shift to cover the whole area.

System Info:

• Distro (incl. version): Ubuntu 26.04
• GNOME Shell version: 50
• Extension version and from where (e. g. EGO, main branch...): 54 (bundled with Ubuntu)
• XOrg/Wayland: Wayland

No source file or test is named in the report. First reproduce the layout on GNOME Shell 50 under Wayland, then trace the rectangle hint calculation for a free top or bottom zone with tiled windows below it. Done means the hint covers the entire available zone in the vertical cases while preserving the working horizontal behavior.