gpm: Margins
3.2 How margins are managed
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Motion and button-press events are constrained to remain within the
visible screen. This means that the 'x' will be within 1 and 80 and 'y'
will be within 1 and 25 when the console is 80x25 cells. However, a
client can keep track of movements outside the screen, by using the 'dx'
and 'dy' fields, which aren't subject to clipping.
The server helps applications in detecting margin conditions by
filling the 'margin' field. Whenever the pointer tries to cross screen
boundaries, it is forced to remain on the border, but a flag is set in
'margin'.
A different policy is in force for drag and button-release events.
In this case the pointer is allowed to go outside the physical screen by
exactly one position. This allows, for example, selecting to end of
line by dragging down-left. The peculiar situation is nonetheless
signaled through the 'margin' flags. The client should be careful to
fit the values within the screen if needed. 
Utility Functions.
Info Catalog
gpm: Events
gpm: Gpm Internals
gpm: Event Types